Actual Space Marines. You can probably get permission to use the code from Robot Army, so it is just a matter of reskinning robots to humans.

Lately I was looking into making a Space Marine player skin but without a good and free model on the market I didn't want to put too much effort into it yet.

If I get one and if I make a skin there will probably be also some kind of RTS like unit system mod coming from me. Maybe.

I think You should check this mod: Enemy Race Manager. There is one support (demo) race based on Terrano from starcraft, and full controlling with Unit Control mod. Army of Space Marines with tanks ,dreadnoughts and other stuff fighting against chaos, space orks or tyranides on other planets (SE) should be awesome (unfortunettly autor looking now for ppls with some skills to help with other race, and we have only zergs, and protos (demo) enemy race and Terran support... but who knows mayby in futhure can we see WH40k race there... or just space marines support army ;)