While searching for bases to raid I've seen lots of gem chests, trees, bushes, etc sitting waiting to be plucked. How about some kind of gem-stealing troop like a leprechaun/troll/Yeti that would have the sole purpose of harvesting gem chests and other potential hidden gems? Make them 20-30 space troops so a player has to weigh the costs of carrying them. After they've finished harvesting a gem chest they become a very powerful fighter.

