I love chess, and I really liked your theme variant of chess, it adds new layer of thinking to already awesome chess.
When you mentioned that queen needs to be earned, I thought we cannot initially buy it (well we can't since we wont enough coin but after some turn we can)
What would have made it more interesting is that, queen would have been locked initially and can only be unlocked after at least one pawn is promoted, that would have increased pawn importance too and would have made it even more interesting.