Some modern / science-fiction recommendations:
Spec Ops : the line
Tom Clancy's End War
Frontlines : Fuel of War
ArmA 3 (the Ravage/Exile mods add more fantasy stuff like undead and 3D object base-building if you want less military-like realism)
Children of a Dead Earth
Rogue System once/if it gets more fleshed out.
State of Decay is cool if you're into this sort of thing.