New types of zombies do come in the later waves. I'm currently on Hoard/Herd 20 and Ive started to see baby zombies (I think), as well as weird highlighted/red zombies. Also after a certain time period, or dungeon clearance, a bigger type of zombie appears, though I haven't seen them in the waves attacking my base.
I also agree w/ the tree stump idea. It is quite annoying to run across a stretch of land where you cleared the trees only to find yourself getting stuck every few meters.