The idTech model is to open source the code (after a couple of years) but keep the content non-free so you still need to buy the game from the original creator even once someone else creates a better version of the code.

This does not give you as much (legal) freedom as opening up the whole thing does but gets pretty close and most importantly removes most of the technical challenges for improving and maintaining the game. It's also probably way easier to accomplish since writing, art and music has even less of an open source culture than software.