A fellow card collector and I have this conversation regularly when we get together at our monthly meetings:
What card set do you want to see next?
With '07 being the 30th anniversary, Topps has to have something nice and unique up their sleeve. But neither of us have heard of any future releases.
What would be nice is to revisit the 3-D cards again, and encompass the entire trilogy. Printing ability has improved greatly since TPM 3-D cards came out in '99 or '00.
There's also the possibility of revisiting the Galaxy series cards.
We still haven't gotten the second part of the Clone Wars season 1 cards, either.