So, I came up with this variant. I don't personally plan to use it, and I don't think that it's necessary, but it's simple, so I figured that I'd mention it. Here we go....
Normally, when you kill monsters in a given turn, you get 1 treasure, no matter how many monsters you kill. My idea is that, if you kill multiple monsters in a turn, then you draw as many treasure cards as monsters killed, but only keep 1 of them, discarding the rest.
(Or, if you like, then you could shuffle the un-selected treasures back into the deck, so that the deck and discard sizes are unchanged.)
This variant works slightly in the players' favor, since it gives them greater control over which treasures the party gets, but it's not terribly game breaking. It's still more profitable to kill 1 monster per turn than to save up multiple monsters for a big attack. (Why take 3-choose-1 when you can get 3 straight-up?)
(The reason why I won't personally use it is because the game is supposed to be challenging, and, in my experience, the players win more than 50% of the time, so they don't currently need help. But, like I said, it doesn't appear to be game-breaking.)
Let me know what you think, especially if you try it out! :D