The about 15 years old free to play Java MMO Spiral Knights just received a PvP map editor.

Submitted maps have to be tested and approved tho, but it appears unapproved maps can still be played by testers as long as the map is "valid".

The editor appears as the option Coliseum Crucible in the left bottom menu after you accept the extra terms and conditions in the advanced game settings. (It may start in the background in fullscreen.)

Announcement: https://forums.spiralknights.com/en/node/124904

Spiral Knights also got a UI update that added scaling options in September. You can now adjust things like health bar/HUD or chat font for resolutions that weren't a thing when the game launched (like 4K for example).