Man,
With the release of Sephiroth and the murder of Geno, I'd honestly be fine with pretty much anybody for the next 3 characters, as long as the final one is at least sorta hype.
Obvious favorite is still Dante (the Vergil itch has been scratched by Sephiroth) -- although with MH: R coming to the switch I think a monster huner rep is much more likely -- which I'm also down for.
A BamCo character -- Sol Badguy, Nightmare, SIegfried, and a Souls rep come to mind -- would also be cool, especially if they get other BamCo songs to go with them (cough Ace Combat OST's) a la Terry.
As far as first parties go, I can see Waluigi or a Pokemon rep happening, neither of which I'd have a real problem with, especially if said Pokemon character came with Pokefloats.
At this point, Doomguy and/or Master Chief are extreme dark horses IMO. We already have a Microsoft character in this pass via steve, and I feel that the Window of relevancy for the Slayer is closing fast.
As far as how 'relevant' the characters for the second pass need to be, I'm conflicted; Min-Min came from an effectively dead franchise (hence why I think her inclusion was more for the sake of drawing more attention to ARMS rather than driving sales for Smash), while Sephiroth and especially Steve had very broad mass appeal. SImilarly to the first Fighter Pass, I feel like anyone could get in as long as there's enough money to be made for both Nintendo and the owner of the character.