I am thinking of OpenMW for instance. Through reverse engineering, they where able to create an open source engine that runs the game with modern features. You still have to own those games in order to play the original levels/content.

Sure for games, which are game mechanic driven there is difficulty in separating if from the content, but in many content heavy games, it is more about the world, explorations, the story, characters etc, than the just the runtime, rendering, physics etc.

In many games the big chunks of the engine is sort of source available already, because they are written in a scripting or managed language (.Net or Java).

Making the stuff that isn’t written in such a language available to the player as well, would be great. Because that would lessen the reverse engineering burden of modders. And the next step would be to open source parts of it.

The reasons for this are the same for many commercial products to use open source lower levels of their software stack and open source their common code as well. Improving your own product by cooperating with others would be great in gaming as well.