A properly done (Not as cumbersome feeling as PR) MW RO-style game would be epic. Vehicles in UE3 are much easier to make, and aircraft are actually possible.(May be extremely unrealistic, but some work would make it fine.)
Tanks, IFVs, APCs, AA, and Apaches would be right up the proper lane.
Throw in good optics, realistic sounds, great models, textures, animations, RO style gameplay, realistic weapons, lots of them, big maps, and amazing detail, and make it fun.
Then you have the best mod ever.