Glad you like the idea! It could make the game even more interesting and longer.
The roles could be reversed here, with differend kinds of missions we could do on Mars:
-Harvesting: sending crafts to Mars to harvest for example elerium to take back to earth (or alien food!)
-Retaliation: looking for, and attacking their bases on Mars
-Research: gathering stuff on Mars to take home and research
-Base: sending crafts with supplies etc to build bases on Mars
-Supply: sending supplies to our bases on Mars, like soldiers, weapons and crafts
While we do this, the aliens will of course try to stop us. Maybe each mission could have a risk of beeing interupted by aliens.
And at the same time of course, we will have to defend earth in the same way as now.
Guess you wouldn't get much variation in the terrain....
Thats true, but at least we would still have missions on earth. And maybe some areas could have a lot of mountains and caves, some be more flat, and maybe we could have some kinds of alien cities.
But yeah, it could be a hard mod to make and I have no idea how to do it. Maybe some day in the future. Or maybe someone else like the idea enough to give it a try