Just go read Raph Koster’s Website.
In my view, it will be the best tribute and article about UO’s 15th Anniversary that you will read.
Some of the highlights include areas that didn’t make it into the game, at least as originally planned:
In one of these pics you can see the faxed-in cover of Computer Games magazine… I had been slaving for weeks over a very cool and much more elaborately built environment called Teratha. It was a spider-worshipping culture that built its city around a volcano. It became the cover image for the magazine article — and was cut from the game a little while later. I was devastated… but we had to make a lot of cuts… I mean, three whole continents were the first to go…! I remember sketching stuff out for the cultures of the Lands of the Dark Unknown — I think some of this eventually became stuff used in the Lost Lands and in Elikki and some other locales in Britannia. So if you ever wondered where these extra islands and whatnot came from… like Ocllo… well, now you know.
Plus the very first expansion:
Origin asked us for an expansion. We had to use the empty space left on the edge of the map. All that would fit was a small chunk that I sketched out to mimic a portion of Ambrosia from Ultima III. We hired a few new folks — Chris Mayer (Faceless), Runesabre (Kirk Black)… I think it was a team of six, plus some of the folks who had quit working on contract, and we made The Second Age in three months.