Well, this is already being done
Some plans for a singleplayer game using a UO emulator engine were developed a little while ago and are currently being re-thought to bring in line with the capabilties of the latest emulators.
However the problem, so I'm told, is that although the engine is perfect, nearly everything has to be rescripted and from the standpoint of the map, even that needs to be remade.
The current UO map was designed for pvm and pvp, both of which are entirely different to the 'quests' in the real Ultima games (When's the last time you played Ultima 8 and decided 'Hey, I'm going to go slaughter animals in a far away dungeon for loot for a few hours'.
In these games the worlds took a long time to traverse, which is quite a task, to say the least, when using a world like Britannia in Ultima Online.
Needless to say, I'm sure you'll hear more on the matter as more information becomes available, this is effectively the first piece of information you'll probably have heard about this project... let's just call it project SP for now
Happy gaming people,
Mr... S...Anon.