While 1st person control of many ground units is possible, it will not be possible yet to act as a FPS leg unit.
Here's a thought. (and i'm sure i'm not the first one to have it)....
Is it possible to have different elements (land, sea and air) running different versions of the same environments (higher res for ground units, lower res for air units) but interacting with each other in the same game?
For instance, take a small map with a high level of detail (similar to what you'd see in say, BF3, or CoD), then put that town or village down inside the larger DCS map, only with a much lower resolution - one that's suitable for that type of play.
If you took, for example, a village from BF3 as an example (but not actually a BF3 map, i'm not suggesting combining maps across different titles to be clear), you could easily place a village like that in DCS, only with a fraction of the detail provided. Same number and placement of buildings, walls and large objects, just that the buildings are hollow, there's no micro-terrain on the ground, and no small ambient objects. What i mean to say is, you could represent a higher or lower resolution version of the same environment for different players using different roles. And the land based players load up the higher-res version when they play, but are restricted to that particular AO when they play online.
I think that's been the limiting factor in ALL combined arms games thus far. Environments that are detailed enough for infantry are inherently too small for aircraft to play realistically in, and environments that are big enough to accommodate realistic aviation gameplay are far too low-res for land based gameplay. I mean, the aircraft gameplay in BF3, and even ArmA2 is far too restrictive. Likewise, you wouldn't want to play as infantry in DCS given the huge, low detailed environment.
I think the problem is that developers have been trying to do multiple things at the same level of detail when different levels of detail are called for. The world is much smaller to a fighter pilot than it is to an infantryman, and this fact isn't addressed by current all arms sims. The fighter pilot doesn't need to see park benches and empty soda cans, so why not have players using different versions of the same environment based on what they're doing? Is there any reason why players can't be represented in each others environments even though they're running different versions?