There aren't many fast villains in general, but it really depends on what you call fast?
Bowser Jr wouldn't be super fast, but would likely be pretty agile if his Papa had a speed buff.
Black Shadow again would likely be pretty speedy given he's a racer, but he'd certainly be slower than Falcon.
Mega Mewtwo will likely be fast too, although the regular form will remain his floatier medium speed self.
(K)Achi of Sin and Punishment fame is another fast character who is a villain (and then a hero in the sequel...), though her chances are ludicrously low
Sadly for balance reasons K.Rool and Ridley will almost certainly be slow characters...
As for the more Rival types some would call villains there are a few options already that are medium speed (Wario/Wolf), and Metaknight who could honestly be considered a fast character.
Slime could also be considered a villain in the loosest meaning (he's an enemy, but he's also a hero, and it's all very complicated), and he'd likely be pretty nimble hoping, and even faster with his elasto-blast.
However the only real option for a really fast villain to rival the speed of some of these heroes is sadly not a good choice... Shadow is not the type of character you really want in this game is it?