*looks at MW2* that is why not to combine them.
There are 2 types of players in those games - those that camp, those that run about like a loon. MW2 wasn't what you are on about, but the play style is exactly what would happen, you would have half the people sat in corners waiting for someone to come past, the others would be sprinting about everywhere. The game wouldn't know what it was trying to be and would fall over. Jack of all trades, master of eff all
I was speaking to someone yesterday about an idea - combining a large scale strategy warfare game, for instance a Total War type game, with a large scale first person war game, for instance Arma.
You would control units and tactics and what not from a top down RTS style viewpoint, but could control a specific member of a specific unit. It could be combined with multiplayer too to makes things very interesting too.
If you see a game where someone is the leader of a squad, even if their tactics are awful, with a good bunch of people with the right amount of communication, they can pull off the bad tactics, and the same would be the case here. What would otherwise be, on paper, a certain loss, would be unpredictable, because it is actual people down on the ground, not AI and number crunching