I've been researching three game engines.
Unreal 1, Marathon and Doom 3.
Take one element from each game.
Marathon 5D space.
Unreal 1 level design.
Doom 3 portal rendering.
Mix it all together and you get a new game engine.
Modify Doom 3 source code to have 5D space like Marathon with Unreal 1 level design.
I modified a Unity Doom 3 visportal project to work with Marathon levels.
Maybe I can modify those elements to work in one game engine.