I suggest BIS start working on Arma3. With it, a new engine. The current one has met many of its limitations (in the eyes of this scrub), most ghastly perhaps the physics handling and collision detection.
In short, a new engine is needed to do away with some of the most glaring descrepancies between real life and Arma - or better, between other simulations and Arma. Try turning a car, dropping a bomb or doing anything else in a vehicle - those things need new algorithms.
I think Arma3 could become not just an infantry simulator, but a military simulator. Right now, Arma2 excels as a simulator for individual soldiers. With a bit of scripting and some modifications, the community has polished the gem even further. The desire is there to keep expanding on this functionality. Unfortunately, I think BIS do not perceive they need a new engine, if they are to provide improved infantry simulation.
What is severely lacking is the will to simulate vehicles. Here is a good rationale, I feel, to promote the need for a new engine. The evidence for this lack of desire is apparent whenever you sit in the Warrior's driver seat, or occupy any number of other vehicles. Most subtly, perhaps, with the moon-like gravity for bombs from aircraft. Armoured units, part to do with the AI, will drive back and forth and turn around like impatient six-year olds.
I argue, however, that to simulate these different methods of warfare demands a new approach. These are not - I suspect - commensurable paradigms unless one runs a supercomputer. Once you're in a ground vehicle, you must be presented with another type of interface than when in an aircraft - and this is separate from the infantry simulation.
A modular approach may therefore be simpler - although I unfortunately don't know jack about making games, so I can't say.
Such an approach would have the game support different manners of handling within vehicles than outside. Can you imagine if we could start our jet's engines like in Allied Force: Falcon 4, use autopilots, adhere to 'real physics', drag calculations, speed profiles, stalling etc? Or similarly in Apaches (collective!) and in Abrams tanks, rev the engine - use GEARS? Or just have a more realistic interface than sighting and looking at... nothing. A helicopter can taxi by adjusting its main rotor; an Abrams does not have 1,650 hitpoints. It's time to leave the compromise of the vehicles behind, and take on a greater challenge. I think this endeavour could produce the highest yield to the developers.
If Arma3 could incorporate enough elements of realism from the worlds of armoured vehicles, helicopters and airplanes, can you imagine the reviews? That would be as groundbreaking as what was launched in 2001. Militaries should desire to use it for training, and Bohemia would gain the absolutely massive audience that is the flight sim/combat flight sim community.
If 'modularity' helps in this regard, I don't know. But it doesn't seem to me as if spit-polishing today's game will see Bohemia into the future.
(btw, this is not for "our Next Generation Game ArmA 2 & the standalone expansion Operation Arrowhead" - it's for a new game. I didn't see any such suggestion-forum, so you'll forgive me if I missed a more appropriate forum!)