Have you heard of or tried Ultima V: Lazarus?
It's a fantastic total conversion of Ultima V using the Dungeon Siege engine. It requires that you have Dungeon Siege or Legends of Aranna installed (won't work with Dungeon Siege 2). The (almost) complete Ultima V with extra sidequests and the option of becoming an evil Avatar. It's one of the best game mods I've ever played. I believe Richard Garriott himself gave Team Lazarus a big huzzah.
some things from the original U5 couldn't be implemented in the DS engine. The blink spell had to be cut, for example.