As far as I can remember, my very first experience with an expansion of any real note was Second Age for Ultima Online, I remember having warm feelings about this type of content delivery at the time given how much it added to the game, but sadly that feeling has evaporated since so many developers use it as an excuse to release unfinished product to the market.
Ultima was also my first experience with MMOs, it's such a shame what ultimate ended up happening to the game once Garriot, and Origin were taken away from the game. I still have many fond memories of it regardless.
I would love a new game set in the Ultima universe if done right, but the fact is that I fear there are not too many developers out there who could really do it the way most would want, and therefor should be left to it's legacy as a series of greatness. Please don't let Bioware touch it.....