I have been summoned.
Fox will remain a permanent member of the Smash roster. Even if the next game has a rebooted roster the same size as Smash 64's, Fox would be a part of that roster. No doubt. If the next game has a roster between Brawl's and Smash 4's size (40-50 fighters), I can see at least one of Falco or Wolf staying, if not both. Falco has been in more Smash and Star Fox games than Wolf, whereas Wolf is a more unique fighter and has "evidence" of fan-demand thanks to the Ballot.
If Star Fox remains in its current state, the only way I see it getting a 4th character would be if Ultimate gets ported onto the next console, and they start pulling from Assist Trophies to turn into new fighters since they already have a lot of work done on them. Krystal obviously did well enough on the Ballot to warrant getting in as an AT, and honestly she's been treated very well for an AT in Ultimate. I can genuinely see her getting in under those circumstances.
If Star Fox finally sees a Fire Emblem: Awakening or Metroid Dread-like revival, I can easily see one new fighter. Krystal's the obvious front-runner, but if she happens to be omitted from the next game again, I wouldn't discount Slippy's chances. He did star in his own spin-off, Star Fox Guard after all. Plus his body type is close enough to Peppy's you could bring him in too as a clone like a two-for-one deal and get the entire original Star Fox team.
And if anyone is interested, I've made movesets for Krystal and Slippy/Peppy. Krystal's takes heavy inspiration from
Super Smash Flash 2 (Where her moveset is nearly flawless) and Slippy/Peppy's is partially inspired by the
Smash 64 Remix mod. You can find Google Doc links to Krystal's
here and Slippy/Peppy's
here.
As far as stages go, Smash likes to stick with spacecraft, so I'm unsure of them doing something different. If Krystal got it, something from Sauria would be nice to see. Again, SSF2's
Krazoa Palace stage is a ton of fun, though not competitively viable. More likely than anything the next stage would take place in an area from the next game, whatever locations it may have. Still disappointed Orbital Gate Assault never returned, but my theory is that is wouldn't cooperate with the Stage Morph mechanic.
Music-wise Star Fox is
very under-represented. Star Fox Zero getting only three tracks is shocking, Adventures having nothing is criminal, and with Star Fox 2 having seen an official release it should get some love too. And that's not counting the other games with
tons of fitting tracks. I could make a huge list, but not to overwhelm, I'll share two from the three games I mentioned. From Adventures:
LightFoot Test of Strength and
Boss Drakor; From Zero:
Titania and
Sector Beta; And from SF2:
Eladard and of course
Surprise Attack.
In the end, Star Fox is in a decent spot as far as reputation goes given its current state. There is
so much more it can offer to Smash, but its got to compete with other, unfortunately more successful series to get any more love. Star Fox as a series has so much untapped potential, and I still pray that one day Star Fox will get its "Awakening" or "Dread" moment and make a huge comeback.