I think CryEngine2 was a good choice.
Big maps is #1 for me. Also, extremely nice graphics.
And since CryEngine3 will be coming out, I am going to guess that there will be a way to port from CryEngine2 to CryEngine3.
So if this mod lives long enough, we might see it in CryEngine3 with new features that can be used from CryEngine3 (procedural destruction, dynamic lighting editor to make cooler lighting, optimized performance, etc)