It would be interesting to have an enemy type for the aliens that is a worm or shark like creature that every enemy it kills it has an increase in size. It could start small like the size of a shrimp and exponentially grow in size for every kill. For general movement of the worm it can either push sand upward for every meter of movement or it can be jumping up and down out of the sand. For the shark it could have a kind of sail sticking out of the sand it indicate its position. For balance it could be that normal rife and up can have penetration power that can hit them under the sand. It could work kind of like a stealth unit for the aliens.
#Alien Enemy Type
hollow pine
icy owl
A unit like a troop transport would also be nice
Or if the big one dies, Lots of smaller creatures come out of it