I was thinking that toward the end of the game there would be a special fountain that was very very hidden, sort of like the one that required the super bomb in LTTP, where you toss in items and get their final upgrades. Anyway if you scoop up water from this one fountain and drink it, you return all the way back to Link's youngest state. It would be a huge secret, maybe a town's person would tell you about this "fountain of youth" if you did something special for him.
Anyway say you start the quest as a 10 year old, and after the first 3 dungeons you are 13 years old, and are at last old enough to wield the Master Sword.
Then you must complete The Hero's 4 Trials, each involving a dungeon. (Trial of Strength, Trial of Knowledge, Trial of Magic, Trial of Illusion) Link would grow one year with each dungeon's completion, and having completed all of them, would be 17 and the worthy of the hero of time and breaking the barrier on Ganon's castle.
After defeating Ganon, a light breaks through the ceiling and lifts him up, igniting him with power. Ganon levitates Link with his magic and chucks him out the window... as Link falls to the sea below, he sees Ganon's castle rise high into the sky, glowing with magic.
After returning to the Temple of Time, Link sees an egg where the Master Sword had been pulled - the sages tell Link he must hatch the egg by undergoing the final test: The Four Trials of Legend: Trial of Time, Trial of Temptation, Trial of the Fallen, Trial of the Gods. One of these trials would involve a dungeon in which running away from enemies caused them to become stronger. Another trial would involve a viscous fight with an empowered being of the past: OniShadow Link!
Having completed the four more difficult trials Link will earn the status of legend and return to the temple of time. The sages would be surrounding the egg as it hatched, into... a dragon! Link would have this dragon to ride for the rest of the game, and could fly it wherever he wanted, seeking out breath attacks such as fire, ice, light, and shadow breaths. When ready, the player would fly the dragon to Ganon's floating kingdom and engage in the final dungeon of the game...
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More Detail of these 12 dungeons
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Four Trials of the Hero
Trial of Strength: Link will traverse a network of caves below the desert floor to find an ancient palace, home to the powerful Collosus of Gerudo. Many enemies within the palace are made of sand or mud, and have a weakness for water... bombing certain walls, pushing certain statues may reveal a supply...
Item found: Gerudo Spear - use while running to poll-vault over chasms. When standing still, Link will poll-vault in place, preventing floor obstacles from damaging him until he tips over.
Trial of Wisdom: In the cold plains of northern Hyrule lies a city made of stone, abandoned for centuries. Within it are statues littered everywhere, petrified by a Basilisk who has covered the area with magic runes which must be deciphered using knowledge.
Item found: Mirror shield - will protect Link from the Basilisk's petrifying sight. By reflecting light through lenses, the Mirror Shield can also generate large amounts of heat to ignite flamable vegetation or explosives.
Trial of Magic: There is a lake in heart of the Lost Woods which only appears under starlight - however there are trees above it's surface (hint: use mirror shield to redirect star light to the surface). A shrine for the spirtis lies at its depth, protected by magic-wielding enemies and barriers... it is guarded by a headless horseman with a shape-shifting magic sword that can take the form of any conventional weapon...
Item found: Magic Hammer - can shatter magic barriers. It's shockwaves damage spiritual enemies immune to physical weapons.
Trial of Illusion A tall mountain in eastern Hyrule has been called too dangerous to explore. While the mountain is said to hold vast treasures, many have died trying to collect them, as they are not all real. The mountain is filled with magma-ridden caverns... there are said to be unseen paths within the lava.
A very large demon hides in the lave, yet no one has ever seen its face.
Item Found - Power Bracelet: Link will commonly have to throw stones into the lava to see whether they splash or break in mid air, to see where the illusionary path across lies.
Four Trials of Legend Coming soon!
There are three introductory dungeons, each containing a piece of the broken Master Sword. Link returns to the Temple of Time where the seven sages tell him to repair the sword, he must undergo the Four Trials of the Hero. By completing the trials, the Hero of Time will be summoned to complete the blade. Until he has passed the four trials, the sage's bestow onto Link the white sword. The sages tell Link that this sword once served a similar purpose as the Master Sword in whiping out evil.
When Link completes the Four Trials of the Hero, he returns to the Temple of Time where a large cutscene shows the Hero of Time appear from the past, to complete the broken sword. Link then grabs the Master Sword and heads to Ganon's castle. As I described before, after a brief fight with Ganon, the evil wizard tosses Link effortlessly out the window. As Link falls through the air, he see's Ganon's castle rise from the ground, floating up into the sky within a field of magic.
Crawling back to the Temple of Time with the Master Sword in his hand, the sages tell Link he must undergo the final test of his virtue: The Four Trials of Legend. By becoming a Legend, Link will awake The Wings of the Gods. The Hero of Time appears once more, leaving in his place a large golden egg with the triforce's marking on the side.
Link completes the four trials of Legend (the final four dungeons of the game) and returns back to the Temple of Time. The egg hatches into the gold dragon which Link can now ride up into the sky to reach Ganon's floating kingdom. The 12th and final dungeon of the game.
There are three introductory dungeons, each containing a piece of the broken Master Sword. Link returns to the Temple of Time where the seven sages tell him to repair the sword, he must undergo the Four Trials of the Hero. By completing the trials, the Hero of Time will be summoned to complete the blade. Until he has passed the four trials, the sage's bestow onto Link the white sword. The sages tell Link that this sword once served a similar purpose as the Master Sword in whiping out evil.
When Link completes the Four Trials of the Hero, he returns to the Temple of Time where a large cutscene shows the Hero of Time appear from the past, to complete the broken sword. Link then grabs the Master Sword and heads to Ganon's castle. As I described before, after a brief fight with Ganon, the evil wizard tosses Link effortlessly out the window. As Link falls through the air, he see's Ganon's castle rise from the ground, floating up into the sky within a field of magic.
Crawling back to the Temple of Time with the Master Sword in his hand, the sages tell Link he must undergo the final test of his virtue: The Four Trials of Legend. By becoming a Legend, Link will awake The Wings of the Gods. The Hero of Time appears once more, leaving in his place a large golden egg with the triforce's marking on the side.
Link completes the four trials of Legend (the final four dungeons of the game) and returns back to the Temple of Time. The egg hatches into the gold dragon which Link can now ride up into the sky to reach Ganon's floating kingdom. The 12th and final dungeon of the game
EDIT: Going into more detail.
Plot of WW2.
In the beginning of the game, as soon as you start a new file, Adult Link is in a burning castle. Flames erupt from the floor and walls, bringing curtains and stone pillars crashing down all around you. Dush shoots out of debris as it shatters on the ground... behind it is a voice screaming "help me Link!" in the distance. The music is a remix of the LTTP castle-in-the-rain song that was also in the beginning. (these rainy scenes have become a classic prelude to many Zelda adventures)
Suddenly you are in control of Adult Link: you run forward, dodging the falling objects as the castle falls apart all around you. Going from room to room, you enter at the balcony of a large foyer, suddenly the ceiling splits open as fire and ash spill through in an explosion of dust. The floor is a sea of fire... choicelessly you swing from chandelier to chandelier across to the other side.
Going further into the castle you eventually come outside where it is raining violently, wind swirling around the night sky as the sea glows with moonlight behind it. You race up several flights of stone stairs with the voice still crying in the far distance... suddenly the stairs begin to split down the middle as huge piece of stone fall hundreds of feet to the sea below. Quickly you race upward, jumping over holes and staring blindly through the rain and dust. Eventually you re-enter the highest floor of the castle.
As Link moves to the back of the highest floor, he will again find a balcony leading to outside. The voice suddenly becomes extremely clear, as Link stops (cut scene)and stairs up at the endlessly high tower in front of him, a huge wall of vines running down it, blowing violently in the wind. Halfway up the wall of vines is a giant man in black armor with a blonde girl slung over his shoulder, climbing up slowly. "Help me Link!" she cries. The man in black armor (Ganon) turns his head down and makes eye contact with Link. He then proceeds to climb quickly as the girl screams.
You are in control again: you run forward and latch on to the wall and start climbing up - the second you do, another quick cut scene shows Ganon kicking off pieces of rock from the wall as he climbs: the camera swings below your feet and stairs up at the sky. As you climb up the vines, which blow chaotically into the wind and barely ever touch the wall itself, huge slabs of rock fall from above you. Dodging them carefully by swinging the vine left to right, or jumping to other vines, you climb higher and high, the rain shooting down like arrows and the sound of the sea crashing violently below you. Eventually about halfway up, another cutscene shows the man climb into a small gold-lit window, pulling the girl in with him. You scurry up the rest of the way and climb into the window. You've only a glimpse of the girl's face before suddenly all the light swirls around and vanishes...
"Link! Link wake up...we're going to be late!" Suddenly you see child Link sleeping in his bed, slowly turning over and sitting up to rub his eyes. Link's uncle sits in front of him and offers him a warm smile: "Come on and get dressed, we're going to miss the Princess's Fair!"
You get up and put on your green Kokiri tunic and go downstairs where your uncle waits. "Remember Link, today is the day all the people of Hyrule will gather at the castle to meet the new princess. It will be a huge, splendid event! There will be games, parties, hundreds of people from all the corners of Hyrule!" He explains it to you as he prepares the both of you some soup.
Not long afterwards, your uncle and you leave the house, going outside to where a horse is tied to a post. He climbs up, then helps you onto the back seat. With a kick the horse takes off, and the two of you head north to Hyrule castle as the bright sky opens with the flare of early noon.
For the next few minutes you see the horse racing through the fields of Hyrule, Link holding on to his uncle as the horse races through fields, forests, desert... the player is offered a brief glimpse of the areas of Hyrule before the quest has really begun. In the background, the traditiononal Legend of Zelda theme song booms with bright fanfare, while the game's credits appear in the sky.
After a few minutes, you see Hyrule Castle in the distance (still a cutscene), a large shadow of several towers and hundreds of buildings. Around it is a large wall and moat, with several hundred horses and wagons pouring into the entrance: quite an impressive CG sequence!
The next scene is top down on thousands of Zora, Goron, Kokiri and Hylians, all parading around the city. Baloons pour into the sky, fireworks are going off everywhere, kids are running around as the sound of laughter and bells chime all around you. It is a joyous fair and suddenly you are in control of child Link again, as your uncle ties the horse up: "Meet me back here in an hour, okay Link? We don't want to miss the Princess's unveiling."
For the next 15 or so minutes you can run around the giant sized Hyrule court at will: everywhere it would seem there is something to do. Games, prizes, events, races... more impressive is the sheer number of people on the street. - there are literally hundreds. Children and chickens run all around you, as the sound of singing and dancing surrounds you and you just...feel like you are there.
When the time comes to meet your uncle, the two of you follow everyone else to the front of the castle. You are now two people in a huge audience in front of the door, which slowly opens as everyone lines up, to show respect for the people who will be walking out.
Royal guards march out on gallant horses of armor with pikes, coming to a stop at the four corners of the city square. A red carpet rolls out of the doorway, as trumpets begin to sound and the audience begins to cheer. Link and his uncle smile at each other with anticipation.
The King of Hyrule, Princess Zelda, and Ganon walk out surrounded by a few nobleman, lords, and a circle of bodyguards. Suddenly the camera zooms in on Link's face as his eyes widen and his jaw drops: it is the girl and man from his dream!
The camera returns to the King, Zelda, and Ganon... Zelda is looking around nervously into the crowd, the camera zooms in on her face. Suddenly she stops when she sees Link... her eyes widen.
End of Beginning
You will learn soon later that Zelda had a similar dream