The ACE2 mod doesn't begin to approach the fidelity and scope of DCS, or even FC for that matter, at least in terms of flight simulation; nor can it, given the inherent limitations of ARMA. Personally, I don't think the integration of a FPS and a high-fidelity military technology sim is worth the attempt. The two types of games operate in completely different domains. FPS games are about representing human actions in a given environment. Military technology sims are about representing the operation of sophisticated weapons systems in combat.
That said, I do think that ground combat can be integrated into a flight sim, primarily in the form of vehicle combat and close air support. For example, I think adding a reasonably faithful tank simulation component to DCS is feasible. Most of the elements are already there. You would need to create a control scheme, add some vehicle physics, create some interior and exterior views, and add some AI, but none of that would involve completely rewriting the engine, like integrating a decent FPS would.