There's actually quite a few--vanilla and chocolate are often the most common flavors, as well as making frosting with cream cheese for some types of cakes. There's also buttercream frosting, and I've come across mentions of some flavors used for cupcakes such as strawberry, lemon, mint or even expresso-flavored for people with particular tastes...

[Though we're pretty sure you don't want those kind of flavors (like expresso haha) for your red velvet cake in particular, but the ones mentioned before are at least more normal sounding to have...]