Well referring to the multitude of licenses available - If you don't mind sharing you get it all when you use a big 3rd party IP - perhaps that's one solution cost-wise.
As for interest - the whole premise of most FPS games rest on attributes, not so much on story-line, unless it's epic so as you agreed you are kind of Archimedes there as far as a full DF style FPS goes.
Another point...
Just because a lot of maps are small doesn't mean we all like it.
Some small maps are very good but IMO most of them are just lazy efforts - a lot of players like to leave the scene etc, you need to cater to the impression of a realistic experience, dead zones and invisible walls make me walk. Once again IMO there has to be an explanation for that confinement.
As you can imagine you need a lot of room to manoeuvre a helicopter, let alone a squad or two of them battling on, it's just ROODOLFF KRAPP joyriding if it's all cramped and over populated ala BF3.
So after all this we are mostly left facing a big optimization compromise, all of the great ideas go out the door - it's no fun when that happens, it goes cold.
So knowing exactly the limitations and compromises ahead of time saves a wasted effort - I've seen it personally and everyone here has experienced it with Angel Falls.
Good Luck
