Okay, finished my gifs, so here are my updated, now much more open possibility-wise DLC Wishlists..I have lists for both Unique Fighters and Echo Fighters.
Sora (Kingdom Hearts) [Disney]
Kingdom Hearts has a massive fanbase and is likely the the biggest name in regards to Action RPG series. Sora, himself, has a lot of moveset potential as well with many moves, attacks, and spells that could be pulled from the 5 games in which he was the main character (6 is Coded is counted). He's also a very, very popular request for Super Smash Bros.
2B (NieR Automata) [Square-Enix]
Considering how popular NieR Automata and 2B are, I feel like she'd be a good fit for Smash..though her under skirt & top may need to be subtly altered akin to Bayonetta or fCorrin. Based on her flashy attacks and abilities in NieR Automata, I think she'd be an interesting and fun fighter.
Byleth (Fire Emblem: Three Houses) [Nintendo/Intelligent Systems]
Byleth may not be as popular a choice as Edelgard, but I believe they would be more interesting moveset-wise and would better represent Three Houses as they are not only the main character but the only real option that isn't tied to one of the houses but to them all instead. With the Sword of the Creator being quite unique in its Chainwhip-style blade, the moveset would definitely be different from anyone else's.
Rex & Pyra (Xenoblade Chronicles 2) [Nintendo/Monolithsoft]
I truly believe that Rex & Pyra have a very good chance of appearing as DLC now that it wasn't all decided early on. I mean, Rex was the only one commented on as being 'too late' to appear. Xenoblade Chronicles 2 was also a surprise success for Nintendo & MonolithSoft and the pair have a lot of possible attacks they could use and a unique gimmick.
Zero (Mega Man Zero) [Capcom]
With the Mega Man X incarnation of Zero already being an Assist Trophy, I feel like the door is still open for the Mega Man Zero incarnation (which I'm biased towards as my fav. Mega Man series). However, with that comes a more unique moveset considering that MMZ Zero has a variety of weapons to use for specials and regular attacks consisting of the Z-Saber, the Z-Buster, the Shield Boomerang (think Capt. America), the Triple Rod (like a lance), the Chain Rod (like a chain-whip..kinda like Byleth's blade), the Recoil Rod (like tanfas), and the Zero Knuckle (a grab that can steal abilities..but here it could just be referenced in Zero's grab).
As a bonus, the Mega Man Zero incarnation of Zero is, of course, the main character rather then a side character/secondary main character as he is in the Mega Man X series.
Dixie Kong (Donkey Kong) [Nintendo]
Based on Diddy Kong. Dixie Kong would complete the trio that is DK, Diddy, and herself. Similarly to Chrom, she could have a few unique moves to compliment her unique abilities in Donkey Kong Country 2/3/Tropical Freeze.
Octoling (Splatoon) [Nintendo]
Based on Inkling. Pretty self-explanatory, just swap out some of the weapons for equivalent ones similarly to Dark Pit and change the stats for each one.
Medusa (Kid Icarus) [Nintendo]
Based on Palutena. Mirror her handedness and change some of the effects and a few animations/attacks and Medusa would fit nicely as a Palutena Echo.
Metal Sonic (Sonic the Hedgehog) [Sega]
Based on Sonic..both conceptually and canonically. Make the movement faster, but more slippery, attacks could be slower but hit harder..that juxtaposition in and of itself could be fairly unique.
Hilda (The Legend of Zelda: A Link Between Worlds) [Nintendo]
Based on Zelda. Completely biased here..but I love Princess Hilda. She could be like Zelda but to incorporate her 'opposite world' nature, her elements could be opposite, such as her version of Din's Fire being Ice Element. Her Phantom equivalent could also be an enemy forming behind her and attacking her to represent her kingdom having a poor opinion of her (kinda like the Zombies from Jill Valentine in Marvel vs Capcom 2 or Frank West in his Versus Series appearances in an 'enemy' assist). Granted, Impa as a Sheik Echo would probably be a better choice.. >.>
I'd love to hear some opinions on my picks..as long as its, well, you know, not rude lol