From what I've seen, here's pretty accurate list of characters I think have the best shot at being Echos.
Donkey Kong - Funky
Very similar body type and was playable in the recent port of Tropical Freeze where he's also very much like Donkey Kong. He was also playable in Mario Kart Wii which sold over 35 million copies so he's pretty well known by people around the world. Plus if he has Funky voice and some meme tastic taunts and he could be a new fan favorite.
Ness - Ninten
I mean Ninten already looks extremely similar to Ness so it's not like they'd have to change his model that much. Plus if Fire Emblem can have Marth, Lucina and Roy then it's not hard to imagine Mother getting Ness, Ninten and Lucas which would literally be the exact same situation. Plus this would then make the stars of every Mother game as a playable character, which would be a nice way to try and please that small but extremely devoted fanbase.
Captain Falcon - Black Shadow
Now that Ganondorf finally uses his sword in all his Smash attacks, combined with the other changes he's been given between Brawl and For, he's pretty far removed from the slower but more powerful clone of Captain Falcon from Melee. Black Shadow as an Echo would be the perfect and easy way to give the Ganondorf fans of Melee that old moveset back.
Sheik - Impa
Impa's design from Skyward Sword and Hyrule Warriors would fit Sheiks built and moveset pretty well. Plus like I've said before Impa would be the most important reacurring Zelda character behind the current cast so if Sakurai wanted to include another Zelda character that wasn't Link, Zelda or Gannondorf, making her an Echo of Sheik would be an easy way to do it.
Roy - Eliwood
The 7th Fire Emblem was the first one released in the West and Eliwood was the main character of that. When Lyn as an Assist trophy and Hector requiring a unique moveset to get in, if Sakurai wanted to appeal to fans of that game Eliwood would be the easiest to get in.
Snake - Big Boss
Well Snake is already a clone of Big Boss in the MGS universe so it would be fitting that Big Boss gets in as a clone in Smash.
Ike - Chrom
Chrom was originally planned on being playable in the last Smash before Sakurai decided Robin would be a more unique character to put in. He's still pretty popular among Fire Emblem fans so if Marth can have his own Echo with Lucina who's Chrom's daughter, then using Ike to add him would be pretty easy to do.
Sonic - Shadow
This one is about as obvious as it gets. He's still really popular with a large percent of the modern Sonic fanbase and the easiest Sonic clone to make.
Olimar - Louie
Yeah I knew Alph is already an alternate custom for Olimar but if Sakurai wants to give Pikmin a second character that's somewhat different, Louie would make a good Echo. Plus he's secretly evil so it could give another villain to the roster.
Villager - Isabelle
Isabelle would be the most popular Animal Crossing character that would make sense to have Villagers moveset as well. If they want a second character from this franchise but don't want to make something unique, this would be the most fitting Echo.
Mega Man - Proto Man
Now this one could be tricky since he's already part of Mega Mans Final Smash. Still, after playing Mega Man 9 and 10 I would love to see Proto Man get in as a more powerful version of Mega Man, but he'll take a lot of damage like the game he was from. He could be a great glass canon character that would be easily to implement with Mega Mans moveset. I mean, they could always find a way to edit Mega Mans Final Smash so if it was used on Proto Man he just wont appear in it. Yeah this one is more wishful thinking on my part but I'll still keep my fingers crossed.
Palutena - Medusa
This one would be controversial because Medusa easily has a lot of things from her fight in Uprising to make her unique. But Kid Icarus isn't exactly the most popular Nintendo franchise and I'd imagine more requested characters would be given the time and attention for unique movesets. Still, since Medusa has the exact same body type and uses a staff like Palutena, if Sakurai wants her to be in just for the fan service like Dark Pit, I could see him adding her as an Echo. Plus it would give the series a female villain, something it currently doesn't have in the roster.
Pac Man - Ms Pac Man
I mean all they really have to do it make Pac-Man faster and add a bow and lipstick. Probably the easiest Echo character to implement.
Ryu - Ken
Reading about Sakurai's past will show he was a huge Street Fighter 2 fanboy back in the early 90's. I would not be surprised if he really wants to add Ken, and making him an Echo of Ryu would be an easy way to do it, and match how he was in Street Fighter 2 as well. Plus Ken is still insanely popular with the Street Fighter fanbase so there's a lot of people who would easily be behind him doing it as well.
Cloud - Zack
This one is probably the least likely but hey, Zack is popular with the Final Fantasy 7 fanbase and even had his own game. So if they want to make the Final Fantasy 7 fans go crazy, Zack would be the easy choice to squeeze another character from that series in.
Bayonetta - Jeanne
This one I wouldn't be surprised if it happens. She's plays exactly like Bayonetta in both games so it's not like anyone will complain if she's a clone of her in Smash Bros since it would literally be 100% accurate to the source material. Plus since Bayo was the most requested character to appear during the DLC poll, it gives an extra reason to give her fans another character to play as from that series.