The number of physical maps can't be changed. You can "clone" any existing land mass into a new facet as many times as you wish inside MapDefinitions.cs. You can also replace any of the existing maps with a new one, either created yourself or downloaded/purchased elsewhere. I'm sure that part is covered in detail somewhere on the site here.
I've used a purchased Darkshard map to replace Fel while keeping the original Tram, something that can't be done on the latest clients. Replacing the other maps is easier. I did have to tweak the housing placement code because the new map has a "rougher" terrain that would prohibit housing in most spots.
ultima live? As i understand it you live stream your map to players instead of them having to download an update or patch, making it possible to instantly change the map. However it sounds like it may be difficult to install and get used to using, and sounds like it would possibly slow things up a bit using more bandwidth.