While being intuitively quite attractive, it doesn't sound that good on second thought - at least to me.
As there are cards which excel in different phases of the game, like novas, towers, cards with growth or ablaze or the already mentioned earthquakes in the early game, and most of the expensive cards, especially miracle, in mid- and endgame, this would bring up a lot of balance issues. It would mean a lot of work to even things out.
Implementing it as an alternative game mode, it would be necessary to balance all the cards in both modes simultaneously, which would lead to a real chaos.
I do like lysol's proposition, though.