I don't know about Jill, but Leon could easily have a moveset strictly based on his appearance in RE4.
There's plenty to work with: he does suplexes, he's got a knife, a grappling hook, different kind of grenades (explosive, fire and flash ones, his grenades in particular would translate to Smash really well without resembling Snake's), medical herbs, a rocket launcher, a shotgun, a mine thrower, an UZI, a precision rifle... and he also throws eggs.
Sure, we've already seen some of those things in Smash, but you put it all under a survival horror theme and in Leon's character and it could come together in a pretty unique way. We haven't seen anything like UZIs or precision rifles for example, and in RE4 you used those things in very different situations that could translate well to Smash. The UZI would be a "get off of me" move, and the precision rifle would be good for something like edgeguarding. He could also have a limited projectiles mechanic.
And El Pueblo would also be my choice for a RE stage. Maybe RE1's zombies are too much for Smash, but the ganados and the lake monster from RE4 aren't really more excessive than some of the Castlevania stuff that already got in, so...
Leon would also be the only fighter with a leather jacket and I think just that's enough to want him in.