Krystal
The thing with Krystal is she hasn't had a major role in like a decade, and Star Fox's last game was a bit of a flop, and she didn't get any more recognition in Smash 4 than she had in Brawl.
She would add more female representation and she could have a unique move set, but that applies to a lot of characters.
I think the return of Wolf is about as much as a Star Fox fan can hope for (and I would appreciate it since I liked playing as him), while Krystal doesn't have much chance. BUT if Star Fox
did get a newcomer, it would probably be her, as she's more compelling as an inclusion than Slippy or Peppy or Andross.
Krystal chances: 2%
It's highly unlikely that Star Fox would get Wolf back AND a newcomer unless we're getting a large roster expansion (and/or significant cuts to open up more slots), but I also think it's unlikely they would choose to add Krystal but not Wolf... Wolf is huge competition for her, and she wouldn't be that likely even if he wasn't a problem for her. There will be much crying in Yiff-town.
Krystal want: 30%
She could be interesting, and Smash is a bit too male dominated. But I would rather have Wolf back, probably (subject to revision upon seeing her in action). But TBH I wouldn't be bothered if they cut Falco, brought back Wolf and added Krystal.
One final note on Star Fox:
And so it seems Star Fox will forever be a sausage fest.
You may or may not be disappointed to learn that Falco (and Slippy) ain't got no sausage. They would have cloacae
Rex & Pyra prediction: 60%
Nominations:
Roy x5