The ioquake3 engine is based on the Quake 3: Arena game, you’ll need that or the demo data to have somewhere to start from. See here for example: Quake III Arena on Steam you can build whatever you want with the ioquake3 engine but it’ll be easiest to make a multiplayer FPS. For example the Quake 3 Rally game is a racing/combat type of thing made with this engine. http://www.q3rally.com and any project that doesn’t use (or has replaced all of the) the Quake 3 data can ship on its own without requiring the player to have Quake 3 as long as the terms of the GPL v2 are followed for the source code. The Quake 3 game data is still commercial data and is not free.