*Gameplay overhauls that add new things, ie enemy behaviors, weapons, weapon types, etc. The FAQ led me to this page, which mentioned the Satanic Gameplay mod, which sounds really cool but I'd like to hear some other suggestions as well.
Currently I am now on the hunt for other interesting gameplay overhauls of the classic DOOM games. let's just say I found out about some stuff and don't much care to use them any more. I like Brutal DOOM and its forks in theory, but, well.