I always say keep it simple, few guns, one map, player model, that's it, but these minimum of things need a particular level of quality, which requires for a solid team to work on.
Unfortunately when I think about it I must agree with you Psychomorph. I also do not think now that modding RO2 or even In Country:Vietnam would be a good idea to create INF3.
The most striking thing that we still miss here is that those games will not be based on modern warfare theme -- that means we would need a *very* considerable amount of coordinated work from many people to create models for modern guns, camos; even maps would have to be completely redone to match modern environments. It would take months at least to do that taking into account there would be a very dedicated and skilled team to do that in the first place.
Honestly I think it would be much easier to mod upcoming Battlefield 3 considering the fact that all models and modern environment would be basically already there. We would only need to add things like freeaim and realistic movement speeds and of course tweak those weapons to match them to realistic parameters.
But it also won't happen because Digital Illusions is not releasing any mod tools for Frostbite 2.
There are some experimental mods for CryEngine that offer freeaim and realistic movement, but gathering them all into one place or rewriting them to create some serious reality mod is not gonna happen any time soon also. I don't know of any serious team who would be working on that in the new CryEngine Sandbox. Crysis is still too much sci-fi especially in the case of character model to really take it as-is into reality mod without *much* rewrite. And I bet they will not let civilians use and mod the work Crytek done for the US Army recently.
So it's very possible to create such INF-like mod in CryEngine, but it would also mean a ton of work.
The same situation or even much worse situation is with UDK. As far as the Source engine the only reality mod I have heard of is Insurgency. And that is basically crap when compared to INF.
We can also wait for Ground Branch and Project Reality 2 (based on C4 engine) but I don't see them around for the next two years or so. Arma 3 in my opinion still will not deliver realistic CQB -- it will be just a more graphically advanced Arma 2.
The overall situation is not very optimistic if you ask me.