Asheron’s call, an older MMORPG, has quite a few private servers and the original company Turbine is okay with it and still offers the main client to be downloaded from their website.

A few have made very custom content with new areas, higher levels, etc on top of 15-20 years of content updates from the original game.

Supposedly 3,000 users online right now but some servers allow botting. https://treestats.net/player_counts/?servers=emulator&range=