Not sure why we're impromptu wishlisting, but sure:
Third Party:
5. Phoenix Wright (+ Edgeworth Echo) - the Ace Attorney series is one of the DS's biggest properties, and Nick has been proven to work in a fighting game with MvC3. He'd bring a lot of variety to the table with his unorthodox origin.
4. MGS2 Raiden (Snake Echo) - Yes, an echo is on my top 5. Raiden is my favourite Metal Gear Solid protagonist and, given the playstyle of MGS2, it'd be easy to port him over. Maybe attach a few swordsmen's moves to change him up from Snake a bit?
3. Kainé - While Nier and 2B are the faces of the franchise, Kainé feels a bit like an unsung series mascot. 2B has costumes of her in both of her major game appearances, and despite being a sword user, her other powers could bring a lot to the table.
2. Raidou Kuzunoha - Probably the most unlikely one on this list, Raidou is my favourite SMT protagonist. What I wanted to see from Joker, and it unfortunately wasn't integrated, was special attacks briefly summoning different demons, and Raidou would be the perfect chance to integrate this.
1. Arbiter - While Master Chief is the more likely pick, and I'd still be super excited for him, Arbiter is who I really want. A Covenant-based moveset could bring so much to the table. Plasma grenades? Fuel rod cannons? Energy swords? Plasma pistols? There's so much that would work.
Honorable Mentions - Shantae, Alucard, Shadow the Hedgehog, Proto Man
First Party:
5. Diskun - He feels a bit like a missing companion to R.O.B. I'd love to see Diskun with attacks based on forgotten disk system games. Plus, his design is undeniably cute.
4. Satellaview - Can you tell I like old peripherals? Being a pair of characters, one of which is able to fly, there's a lot you could do with the Satellaview mascots.
3. Team Rocket - I'd love a foil for Pokemon Trainer. Jessie and James sitting it out in the back, with Meowth, Wobbuffet, and Arbok at their fingertips.
2. Ashley - I don't think I need to make a case for her, she's been one of the most wanted first party characters in the Smash community for a while now. There's a lot she could bring to the table, for sure.
1. Gothitelle - If you didn't see this coming a mile off, you need a new pair of glasses. Gothitelle is my favourite Pokemon, and while I know her only chance would be as an echo for Gardevoir (good times), I'd still absolutely love to see her make the cut. Very much my never-ever.
Honorable Mentions - Zapdos, Geno, Marx, Vaati