I would love to do a Martian Empires setting. I think the Martians from Black Hat are wonderful, I have not seen the Highland Studios ones yet.
For the humans there are many options, any colonial figures really. As for armour and the like All Quiet on the Martian Front will offer some ideas as does Spartan Games Dystopian Legions, I know they are 28mm but some of the vehicles would make very good large vehicles.
Any other suggestions?