i like this idea, a while back i had a thread on the subject (in game suggestion forum?), basically because my favorite game of all time is ogre battle, which has branching upgrades (ie fighter can turn into a knight or samurai etc).
to me, the concept of leveling up cards from basic > upgraded selection (and possibly even more upgrades thereafter) would be a great start for the basis of a sequel to elements, as if this were implemented it would basically be an entirely new game.