XCOM. Make it a hybrid between the original and the 2012 reboot. Better graphics and UI/quality of life improvements from 2012 coupled with the scale of the original. I really like the tone of the reboot, I think it has more personality than the original, but the original was better due to the amount of control you have over how you play. I'd scrap soldier classes and would instead have a system where you choose from a number of perks when a soldier levels up. This way specialization would develop over time at higher levels rather than from the first level. Mod support would be a top priority.
The biggest addition, however, would be that you could also play a campaign as the aliens. The game's campaign wouldn't just be XCOM responding to randomly generated alien activity, it would be the alien player doing something to further an actual objective and the XCOM player trying to figure out what that is in order to counter it. At first it might seem like the aliens are just doing random shit but as XCOM gains more info through research and experience they can understand the kinds of objectives the aliens have, then make meaningful choices to thwart them.
From the alien perspective you would see a lot of early success in the various regions you're operating in...but then all of a sudden you'd be facing some serious dudes who are very much sick of your shit. When aliens lose a battle XCOM will gain intel that makes them ever tougher to fight. You'd start to see a more coordinated response to your presence as the game went on.
The aliens wouldn't focus as much on individual soldier development, instead they would upgrade their logistics to support better quality troops while trying to learn more about earth in order to take it over. You'd start off deploying Sectoid teams in smaller UFOs but after upgrading your Mars base, and some other shit, you can start sending in teams of other aliens in larger ships to accomplish victory conditions.
Victory for XCOM is destroying the alien base on Mars, victory for the aliens is subverting enough countries that earth capitulates.