Smash Brothers has been a game that has relied on a few key elements in its game design. These elements have made the game unique, but has also limited character recreation and ideas.
One of these elements is the triple jump. More than half the characters rely on a triple jump mechanic to save them from being knocked off the arena. I would more diversity in character design that moves away from the need from every character needing a triple jump.
Several Heavy Hitter characters don't really deserve a triple jump. It doesn't hit there character and they could create a better mechanic for this gameplay. Examples: Bowser, Donkey Kong, Gannondorf.
These characters I would argue would be better made without the third jump. To balance this handicapped I would make them very hard to Smash, and very powerful in attacks. Bowser shouldn't be a character strong in the air, or having a great come back ability. However, he should be very powerful on the ground.
And I guess that is what I mean. Perhaps creating type of characters.
Standard Smash Characters: Marios and such what will retain the triple jump aspects.
Floating/Flying Characters: These characters will have advantage in the sky, but are weak against Smash attacks and could be knocked clear out...and they won't have as many advantagous on the ground. Examples: Pit, Kirby, MetaKnight, ect.
Tank Characters: These characters very strong on the ground, and very hard to push around. They can absorb tons of damage before being knocked out. They won't have a third jump, but would still have a second jump. Either by not having a double jump, or not having that special Upward attacks.
Speciality Characters: Certain characters play by in a very unique fashion...and they should stand out. Nes, Ice Climbers are examples. Perhaps future examples could be Jill from Drill Doser or Olimar. These characters may not fall into any categories above, or may fit into several.
Now take those classes and determine which characters fit into what...assign speed and damage levels and you can create a much more varied number of combatants. Moving away from the third jump as a neccessity and speed as a necessity will help create variety in characters and a more fleshed out experience.
Air Combat: With the inclusion of more air based characters, Air Combat needs to be enhanced. Specifically, Air Dashes need to be implimented, and Air Combos. Air Combat needs to become an intrigal part of experience. I would replace the Air Dodges with Air Dashes that still dodge incoming attacks, and can help set up attacks from behind.
8-Direction Item throwing. Grounded characters need to be able to throw items in 8 directions to help combat against air opponents. As well some of the guns should have 8 way directional shooting...but not all of them.
Interactive Environments. It would be nice if there is more interaction in the Environments. For example, perhaps Donkey Kong's levels have vines you can grab onto and climb underneath the level. You can still fight while holding onto these vines, but it can help protect you from being knocked off. As well two vines on each side of the level could constantly be swinging for you to grab.
Real environment interaction like this will create more dynamic situations, that don't rely on just moving the level around or environmental hazards for changing the experience.
More powerful throws. I think throws need to take a bigger role the game this time. Smash attacks should be used to knock highly damaged opponents around, but Throws should be able to do the same amount of toss no matter what damage level. How far you throw opponents will be based on that individual characters strength and who they are trying to throw. Bowser should be a character you fear to be thrown by, and Equally you shouldn't be trying to throw him.
Enhanced Adventure mode needs to focus less on combat and more on designing a level that fits each franchise. If this means creating mini-game type levels that recreates the game the characters are from so be it. It will make the game engaging and interesting.
Collectable: I would love the collectables in the game to be used for something. I suggested an online Smash Brothers Collectable Card game. However, anything would work for me. So long as the each collectable I gather enhances that experience more, and gives me a true desire to collect everything.