The open source Zircon engine ("DarkPlaces Mark V Project") is a modification DarkPlaces engine with a mouse driven menu everywhere, ALT-ENTER fullscreen/windowed switching and 88 other enhancements to support user/mapper/mod coder ease-of-use and optional conveniences for those interested in a "1996 Quake" experience. "csqc_full_width_height 1" in console when running Arcane Dimensions at this time It is hoped that some/many/all of these enhancements will eventually make it into official DarkPlaces. The Zircon engine seeks to keep full compatibility with DarkPlaces so either engine can be used interchangeably.
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Best Quake Engine Fork I've used.
Main Developer is Active and is good about responding to User Feedback.
It's easy to develop for, more lenient with Entity Placement than regular DarkPlaces, many user QOL features such as Mouse Driven Menu, easy to build with Visual Studio if you need to make Engine changes for your Mod/Game.
Haven't tried the engines more advanced features but it also seems to be much more Modern in terms of Map Format and I believe the maximum possible Map size is greatly increased.