In regards to this talk of having an overhead view for commanding soldiers, or for walking around and building up your base, you know what I'd like to see? Just once, I'd like to see a game where you aren't the big man. You're not the sole hero of humanity, you're not the big boss of the World Defense Legion Ltd., you're not even all that important. You're just another grunt who is sent into the front lines with horribly sub-par equipment to start hunting around in hedgerows for that psychic alien with his plasma rifle and alien grenades and stronger-than-steel plastic armor.
My view of what would be a fun X-Com FPS is where you're suited up and sent out in the skyranger. You can't control where you're being sent, you're just a rookie. You can't order your fellow soldiers around, you're just a gun with legs like the rest of your group. You don't have a say in what HQ does or doesn't do, you're the fresh meat that's handed a rifle and sent straight into the grinder.
You need to follow the orders handed down to you by your CO, and you'd better hurry up or else you'll have more to worry about than the damned aliens. If you perform well, you'll get promoted. You get badges, prestige, honor, and maybe even some armor. Keep going and you'll eventually make your way up to sergeant so you can boss around a couple people and make sure that shit keeps rolling downhill. If you're a really valuable asset to the organization, you may even have a say in what weapons you get to hold on to. Congratulations. Now go clear out that ethereal command room, scum.
If you fail and are horribly murdered, we'll put your name on a plaque. There will always be another greenhorn to take your place on the next mission.
What we're looking at here is something where you're playing one of those meatshields we all love to use in the beginning of the game. You'll be sent on missions by the AI, along with all the other AI-controlled soldiers you happen to be teamed up with. You'll be ordered to peek around that corner, to open up that door, to make your way up that set of stairs, and to charge into the wreckage of that UFO. As you perform well, you'll be given better equipment to keep you alive, and you may eventually get your own squad of 3-5 soldiers that you can order around in your vicinity. You'll still take mission commands from HQ.
If you survive through all that, you'll end up as one of those flying heavily armored supersoldiers that tears through the alien scum with his heavy plasma. You'll get sent on the more dangerous missions that require your experience, and you may even be picked for the elite squad that gets sent to Cydonia to strike the final blow.
But even if you are one of the "Best of the Best", you're still not Alien Enemy #1. You're still fighting alongside all the other experienced and decorated soldiers that may very well have been fighting with you since that very first crashed UFO in the farmyard. You're part of a team that does the work, and you're governed by an organization that gets the credit.
All that would require a rather powerful AI, however. I really think that a little more focus could be spent on upgrading the so-called "intelligence" in games today.
Bah, listen to me babble on... I do think that a good X-Com FPS is possible, but I personally wouldn't want to deal too much with the base construction or the overall team command if I were in such a situation. It's a pity that I get infected with "dreamer syndrome" whenever I try and illustrate an idea I think could work.