MMO is not gonna be something you'd easily achieve on Unreal's Engine, but the RPG aspect could be done easily enough.
Just making a large map that is fully optimized and such to be efficient as possible and allow for as many players you can fit in UT2004 (32?) might work though.
You are probably going to need to figure out NPCs in some way, for that I'd recommend Old School Monsta Tools.
And you'd also need t ofigure out how to interact with the environment, which I figure would probably be mainly handled in making the map.
So in short, its going to take a lot of time mapping, and might not exactly be an easy task to handle by yourself. This would be an interesting thing to see pulled off though.
Of course, if you use any of the resources I mentioned earlier for your release, make sure you get permission.
I wouldn't recommend using the RPG one as is though, its not very fitting. I'd just use it as reference.
