In iD games, there's a clean division between game-specific logic (which is usually in a separate DLL, haven't looked too closely at Doom3 yet), and framework/e

In iD games, there's a clean division between game-specific logic (which is usually in a separate DLL, haven't looked too closely at Doom3 yet), and framework/engine code. On Quake 2, you could replace the DLL to switch between the regular game and capture the flag. An expansion with new mecanics could, theoretically, be done with only DLL code and new assets. You'd need to change the engine code only to add new kind of effects that it didn't support yet. So yes, if you don't change any code, you'll only reskin Doom by creating new assets (it's usually called Total Conversion). If you want to add new game mechanics, you have to write some code.