my first suggestion is to read up on A LOT of classic as well as new military techniques, they can give you a LOT of ideas on how to program your AI in a combat game
i could probably program every aspect of this with only a little studying BUT my computer isnt capable of much at the moment (wont even run the original doom...i got starcraft running...that's about the heaviest game ive made run, and that was blind luck)
do you want to build a game completely yourself, graphics and all, to the point where you have to create base scenes that are modified with each new level (or code each new level individually), or do you want to use a game engine, like blender3D to simplify the thing?
there are a lot of options here, but basically, what you want to do is create a game where after a period of time(testing, making popular enough, and developing a simple enough engine), can be added upon by the users, which has simpler graphics...you're pretty much creating
Doom, except more betterer?
pretty much any game that comes out today has the ability to be modd'd like that, if the person tries hard enough...you'd have the advantage that yours would be built SPECIFICALLY for that, and thus making it simpler to do so, and it could be a LOT of fun to program, but you're going to wind up having to either 1: have servers for people to play on, or 2 make every computer with the game installed into a server and client, the second is rather resource heavy, and usually somewhat vulnerable, the first requires a decent user-base (PR can be a BITCH if you dont know what you're doing)