What frontiers are left to conquer in Ultima Online?
I dunno. They could actually finish some of the plots that have been running for years? Like adding in Minax as a boss so we can finally finish her. She's been "Up to no good" since the beginning yet we never see her. I mean come on already for the love of god.
UO is the worst game ever in terms of continuing and finishing it's story lines.
As for places, the 8th anti-virtue dungeon, The Stygian Abyss still needs to be added, if they EVER get that expansion out. Then, they can open up other Ultima universe areas: The Underworld, The moons [Trammel and Felucca, yes the actual moons, not the current facets], and Pagan!
I've been wanting to go to Pagan in UO since forever.