Honestly, no. At least not that you can play right now. Star Wars Galaxies was something of a spiritual successor, but is defunct. There have been a few attempts at more free-form open world type games since then, but IMO, nothing really comes close to UO. I've had to give up on MMO's because it all just feels like an experience on rails that is a time sink for some virtual items rather than the kind of free form world modifying choose your own adventure that UO was.