Because cakes are my hobby and my business, I don't use cake mixes very often (except for cookies but that's another story altogether
). Kades, your ideas make me WANT to make a cake from a mix to see how good it tastes.
If it is a chocolate cake, try adding melting a square of baking chocolate (or 1/2 cup of chips and stirring that in, or even some cocoa. For a white cake I agree with the vanilla. Experiment with things that will heighten the flavour of your cake without changing the structure of the mix too much (making it dryer or too moist).
Just have fun!