I say include all three so we`re all happy. It`s not impossible for each to have a different moveset; whereas Fox was primarily a speed fighter (with one projectile and a projectile counter), Falco could be much more projectile-savvy, or at least focus more on ranged attacks/anti-close range for his specials. I`ve heard Wolf suggested for power as opposed to Fox`s speed; I`d say that could work, perhaps with a chargeable blaster like in Assault or even one of those shiny red rocket launchers. He would of course be slower than Fox.
Krystal has the best chance for diversity if she uses her staff and all of its magical powers. She could be a speed fighter like Fox, but with more emphasis on magic of course, perhaps like a combination of Fox and Zelda.
EDIT: Of course, if it has to be one, I`d prefer Krystal. Falco would also make a good Assist Trophy, while as for Wolf...Wolfen Boss, perhaps? Still, if we can have all four, I say so be it.