A baby form of Lapras, Scyther, Pinsir, Heracross, Zangoose and other Basic Pokemon. I can only imagine what a baby Onix would look like.
I agree with the Dialga and Palkia's orgin formes. It would make the trio even more alike than they already are.
A north sea and south sea Shellos/Gastrodon. I don't know why, but I thought it'd be interesting.
Other Eevee evolutions [they will probably give us more in Black/White, they have in other generations.]
And finally.... Mewtwo in his gear from the first Movie!