and maybe random events?
Marines mowing everything down under god's eyes with M4's may get repetitive.
I'd like to see a "survival" RTS. Like, you control the Mayor of some Polis and future marines invade and you must keep throwing bodies at them, placing traps, escaping to different parts of the city. Maybe you're goal could be to revive some ancient god to come and kill all of the invaders, you just have to satisfy some weird ass requirements (some god demands 500 goats). Something something.
Or, then again, marines from the 19th century?
[Edit] That's all from the Roman perspective of course. If you just had a battalion of marines (dunno how much that is) it would be challenging, but still, realistically - I think - easy.