Plaguebearer

Same with a water-based facility (pumping station, etc) - one end could be pristine, centred perhaps around a control room type locale for the 'base', gradiating towards more rusty and grungy decor, dripping water, pools of stagnant liquid, until the opposing side's pumping equipment sewer hole base is reached.

obligatory flood traps, etc.

A lot depends on the static meshes available to you. You could go for fire vs ice theme, the classic red vs blue, you know. Or an 'organic' vs 'tech' theme, though organic shapes sometimes are more trouble than they're worth. Maybe a 'chess' type theme, black and white marble with gold and silver trims... and so forth. i dunno what your basic ideas are, so it's hard to elaborate on them