Specifically speaking, type combinations. So far we've had dark, ghost, psychic, fairy, poison, steel, flying, and ground. We're missing normal, dragon, rock, bug, ice and electric, not counting the basic 3 obviously.

Do you guys think we'll ever see all of those? I imagine a dragon or ice starter is just a matter of time, but I can't see a starter evolving into a bug, much less into a normal type.