I'm not trying to start any kind of fights or anything, but does Star Fox really have that many options? I feel like the majority of its characters -- and I mean a VERY large quantity -- would just lend themselves to fighting like Fox, unless we did something like specify Star Fox Adventures Krystal or took some next level creative liberties with Andross. I mean I'd be down for new Star Fox characters as much as the next person but I don't think the cast is all that diverse for smash viability.
What I feel is often overlooked is how much untapped moveset potential the series has to offer. Star Fox Adventures, Star Fox: Assault, Star Fox Zero, and Star Fox Guard all offer a ton of material for potential movesets.
Krystal would likely have a moveset based entirely off of Star Fox Adventures, making use of the staff. The staff was the focus of gameplay in Adventures, being used both in combat, and to move about the world. The abilities in the game can be easily brought over into Smash, and would fit in quite well. I made a
moveset for Krystal a few months back based around the staff, if you're interested in learning more.
Slippy on the otherhand could pull from Star Fox: Assault, Star Fox Zero, and Star Fox Guard. Assault features many gadgets and weapons that'd be useful standard attacks, Star Fox Zero features a few things that'd make for specials or a Final Smash, and Star Fox Guard offers the AegisCam sentries that could potentially be the focus of his moveset. Plus, he's the team's mechanic, so use of tools would make a lot of sense for him as well. I also made a
moveset for him that focuses on all three games.
Take these two and their potential in contrast to when Fox was made in Smash 64. Back then, there wasn't too much to go by, so Sakruai improvised. You could argue that some of his moves are reminiscent of the Arwing's abilities in Star Fox 64, such as his reflector being similar in function to the barrel roll, or Fox Illusion similar to the boost. Otherwise, his moveset is an original creation.