I've seen a platformer mod already and there's also a RTS mod, top-down squad-tactics mod. I don't think a full-scale RTS with 100s of units at once could be made with the engine, but you never know. Sorry, too lazy to find them for you.
About the visportals, Quake 4 has distance-culling portals that force an area to be blocked even if there's visibility. It will even fade to black nicely for you, free of charge. To my knowledge, Doom3 never had this and it's pretty important.
Func_Portals can be closed at pre-defined distance and some mappers at The Dark Mod have adopted the practice of closing Func_Portals while in
view and covering the closed portal with a patch textured to approximate the view. When 7318's skybox code is merged-in this will be even more effective
as the patch could a skybox material (etc).
In fact, I was thinking that LOD would be more effectively implemented to integrate with Doom 3's culling methods by having hard-coded portal sky boxes
where LOD level maps are kept.