There's something similar to what you're describing in Ikariam (which is a GF game), the Barbarian villages. They're located on every island, and not in a spot where a player could colonize, so they don't take up "player" slots. You can attack it, but if you keep attacking the same one, it "grows" and becomes much harder (over time) to attack, and requires more troops to defeat. (The larger/harder ones also yield much more loot than the lower ones.) Since something like this already exists in a …