Here's some information about my Quake 2 modifications which I worked on a few years ago. The project is almost frozen. 

Goals

• overall improvement of the game engine
• performance improvements
• to provide more eye-candy picture
• to support content from other popular Quake-based games
• new features should not break compatibility with existing game mods and original game servers

Features

• fully network compatible with original Quake 2 - "old clients" could connect to "my server", and "my clients" could connect to "old server"
• fully compatible with mods for original Quake 2 (there's no changes made to the game.dll interface)
• added some funny gameplay features (hardcoded into the engine) - like anti-camper sounds
• visuals:
• support for Quake 3 shaders (with some extensions)
• realtime directional lighting for meshes
• fast dynamic lights
• lightflares (configurable individually for each map)
• significantly better quality of .cin video playback 
• content:
• ability to play with maps from other games - Quake 1, Quake 3, Kingpin, Half-Life (engine performs on-fly conversion of the map format during loading process, entity descriptions are also preprocessed)
• ability to use Quake 3 player models
• user interface:
• better player selection UI
• level browser UI
• displaying screenshots in savegame browser
• customizable keyboard bindings UI
• displaying Quake 3 levelshots and progress indicator during map load
• system:
• much faster content downloading
• the game works nice with large FPS (original game has troubles with this), added possibility to work with very small timescale (timescale 0 will freeze the game)
• support for .zip paks, archives could be "mounted" (Unix term) on the file systems at any time
  example: "mount ../quake3/baseq3/*.pk3" will mount all paks from Quake 3 located nearby to Quake 2
• fast fixed pipeline OpenGL renderer with extensive use of multitexturing (has paths for NV, ATI and ARB)
• a lot of debugging features
• and many more ...

Things yet to do

• finish Quake3 level support (there's no support for curved surfaces yet)
• there's some problems with mesh lighting on Half-Life maps due to removed information about surface lights
• there's a problem with some large Quake3 maps (especially CTF maps) - they're not fit into Quake2's -4096..+4096 bounds (fix will require modifications of both game.dll and engine, so game mod compatibility will be lost)
• probably add support for Unreal Tournament 2003/2004 player models (either directly use ukx, or psk, or at least some way to convert ukx to my own mesh format usable by Quake)
• even more improve mesh lighting
• probably add some GLSL effects (bloom, reflections, better lighting ...)
• possibility to add fog regions for pre-Quake3 maps (with text config)
• add Quake3-like portal support
• make UT2004-like shadows (blob or blured projection)
• add resource management - should free unneeded resources on map change

Downloads

Installation: extract .zip contents to Quake 2 directory. 

Source code

The source code is available on GitHub

Screenshots