I like the idea of a simple experience concept for troops that survive combat. That reminds me a little of the old Bungie game, Myth, where each individual unit would get slightly better for each kill they made and each map they survived. It was small, but made a difference, and gave the player pause about whether to throw away a unit or to guard them more closely. (it might even be neat to keep track of how many games a deck hero unit has survived; even if only for fun) That would also give the player more things to decide as the game draws on.
That’s something that I think should be important at the end: player choice. There’s not a lot of decisions to be made once the table has turned in the player’s favor. Blight games seem to start with a bang, but end with a whimper (which is kind of the opposite of Magic the Gathering).
Having powerful spell cards in the player’s deck might help. In addition to high powered but expensive units, there would be some high powered but expensive spells (perhaps they cost both mana and gold) which could drop a meteor on stack of zombies, wiping out all neighboring units. Something like that would have to be expensive, but to the point that you’re not likely to have enough resources until the end of the game, and allows you to wipe things up pretty quickly. Another similar spells could be something like a teleport, to jump a friendly stack right into the action. Or, perhaps a time warp spell to speed up the game clock. These could also be “artifact” cards rather than spells, or something like that, so that different “game ending” cards could use different resources (gold, mana, or valor), giving the player options in case they end up with lots of one type of resource but not the others.