Those are mostly the things that we need new concept art of. We have several alien buildings already in-game, and a few are still missing, since they haven't been given concept art yet. These buildings are the booster, hive, barricade, and leech. To give you a quick rundown of what each does, the booster is something that oozes out poison for the aliens to coat their claws with, the hive houses swarming aliens and is an attack structure, the barricade is a defensive structure that acts as its name suggests, and the leech extracts resources from the ground.
Of those, which sound the most interesting to you? We can go further in-depth if you pick one (or more).