Pure tower defense: progress by killing biters, dense nest placement, periodic waves with bosses, Master Ammo Chest system. No pollution attacks, only evolution.
Large total conversion mods.
New ways to deal with enemies, be it attack or defense.
Map generation and terrain modification.
Ammo chest doesn't register in the system when placed by bots. (Used fast start mod to get bots early.)
Might be the same for turrets.
