I spent a good amount of years working on a project with a killer team called POLServer. It is an Ultima Online emulator to put it simply. To put it more accurately, it is an offline/online server allowing you to not only control the world you are playing in, but it even has it’s own scripting language built in on the server side to customize it in any way you can imagine. That even includes Shadow Realms, which are akin to World of Warcraft’s Instances.
It is still actively developed with some of the great people I worked with, and has been around since 1999. I started on with them as an actual developer around 2003 or so and stayed with the team for quite a while. It supports most of the features that players have enjoyed from Ultima Online: The Second Age® to more modern times. POL is the most flexible and customizable UO emulator around, period. Aside from it’s custom scripting language written just for it, you can even customize the maps for server and client among many other client files and the server takes that information in and uses it as well. Then there is Packet Hooking, allowing the server admins to create custom hooks for the packet communication between the server and client.
I worked on many things within the server package over the years. From shadow realms, housing, battle systems, to elemental damage and resistance, to even a lot of work put into the Packet Hook system.