This idea is good, but my one concern with this is that the game will end up being too similar to Hearthstone. Maybe that's what everyone wants, idk. My original idea was that there are a couple of ways to lose (and win if you flip them around):

1. If you run out of Summons to use, you lose (so basically if all of your guys are killed).

2. If you discard whatever Rainbow/Epic card you have in your hand before playing it, you lose. The idea behind this one is that all players are required to bring a Rainbow card in their deck. Rainbow cards are really powerful but if you end up losing it before it can be played (either from discard or some other means) then you lose, making for some interesting scenarios.

3. Some other possible win condition? Could do something like Pokemon with the prize cards and all but in a different way possibly.

Any ideas? These ideas are without incorporating heroes btw.