My top 10 (which I'm sure it won't match at all with the new dlc characters). I don't have any specific order, except Isaac, which is the one I care most.
1) Isaac.
I don't think I have to say anything about him. Great potential moveset, good design (but I would like more a mix with his DD design in terms of clothes).
2) Andy/Any AW character.
I hate the strategic games but I comprehend the legacy Wars has. Even if the series has never sold a lot (none of this entries hit 1 million), it was active for 20 years, even if half of them were generic entries. IMO, it deserves a spot (and a new opportunity). Also, I visualize him as a minion control character, with using NES soldiers as a special attacks, like King Dedede in Brawl, but with more control and more useful.
3) Dixie Kong.
Using Diddy Kong as a model, she can be easy to do. She could be an easy semi-clone, changing the special attacks to be more original and relatable of her appearances.
4) Any Zelda character.
Just anyone. Ideally, I would love one Zora (Mipha) and one Goron (Idc which one) just to represent the most important races in Zelda games. But if they included Skull Kid or Vaati, I wouldn't complain at all.
5) Bomberman
The only 3rd party character I wouldn't raise my eyebrows and it's feasible to be a playable character. I'm not fond of its games, but Bomberman is a series heavily associated with Nintendo since it was born.
6) Any Rhythm Heaven character, except Karate joe.
Like Golden Sun (well, even more I think), Rhythm Heaven/Paradise has a strong presence as a Nintendo franchise and has been successful with great sales.
I would enjoy more Chorus kids, Marshall or Rhythm girl, and a rhythm attack character would be interesting (but it would be horribly in the current net). Karate Joe, apart from being ugly visually, it could be a brawler character, which there're a lot of them in the current roster.
7) Ray MK
Mii Gunner is basically him in terms of moveset lol Also, I like his design.
8) Masked Man (Mother)
Yeah, a strange pick. But I don't like Porky at all and MM would be another villain and it could be an interesting character combining PSI energy, a light sword and a pistol/cannon beam.
9) Elma - Rex with Pyra/Mythra.
A character with a double swords with guns with a great mobility? Yes, please (though it would be propably like Bayonetta) I prefer Elma over Rex with Pyra and Mythra, but the other ones are also cool and I wouldn't complain about it if they included. Also, they're more recent and more popular.
10) Any ARMS character.
The last one. A "pure fighting" game (even with its gimmicks) which was one thing Nintendo lacked since... Joy Mech Fight? Normally I would agree with Spring Man, but Min Min or Twintelle would be better, because I find Spring Man a bit boring, design wise. But he's the mascot, so I guess he would be first, with Ribbon Girl as an echo and/or costume.
But, as I said before, I think they'll go for the 3rd parties for the new characters, like season pass, with maybe one or two promotional Nintendo picks, which it's a shame for me, considering I like more Nintendo characters picks.