UO Outlands has been pretty popular for a few years. They’ve implemented a lot of custom aspects (specialties), craft, new dungeons, land, as well as weekly quests and events. My brother and I play it a few hours a week. It’s incredibly popular for the nostalgia and its player base seems to be pretty consistent. It reminds me of early UO where your exploring, learning, and dying a lot. And there tends to be players everywhere you go.
A decade or more ago I ran a custom shard with a custom map and world. There was no “map”. No guide. No quests. Tons of dungeons and bad guy areas. Shops. Towns. Etc. I also contributed to RunUO. I’m a fan of UO:Renaissance. I still have the CD and the old statics files.