I think Krystal, Ashley, and Lyn have pretty good chances as well.
As much as I want Lyn to take Ike or Corn's spot in future games (adding her and a 3H rep would be very fitting, I think), I really don't think they're going to add another FE character to Ultimate. Unless they're from 3H, simply due to the surprise runaway success that game has enjoyed.
My girl Lyn more than deserves a spot in Smash, but unfortunately it wouldn't be good business this time around. FE fans would be upset that it's not Byleth or Edelgard, and Smash fans would be upset at another FE character. Fingers crossed she's a base roster character in Smash 7 though!
If an AT is promoted (which, honestly, is looking less and less crazy every day), it would be Shadow or Waluigi. I'm just looking at it from a business perspective, and as much as I would honestly, truly love to see Skull Kid or Midna or even Isaac playable, I personally can't see them being financially viable choices for DLC.
Still, hey, significantly stranger things have happened with this franchise.