Doom Aeons Of Death
There's Hard Doom too, which Complex Doom riped-off completely.There's also Weapons of Saturn a fav of mine, Faspons, an enhanced WoS fork focusing on a more harder and faster gameplay, HXRTC Project, a custom WoS fork with plenty of enemies and weapons and a shoping drone.There's DoomRL Arsenal, tons'o'weapons'n'enemies, with various classes, Trailblazer, Russian Overkill and Guncaster by PillowBlaster, worth checking, Samsara, Doomguy joins forces with other 90's FPS heroes and there's way too many more to list here. Yes it counts because no it wasn't. Any wad with a DEHACKED modifies behaviour, and Batman certainly did.I wouldn't count Batman Doom or Demon Eclipse because they are not mere 'gameplay mods', they are full-fledged conversions, the new or modified behavior is meant to go with new levels.Granted there are a few oddities. Things like Pirate Doom or the Wolfenstein 3D TC are meant to be used either as their own thing or as a gameplay mod for normal Doom levels.
How can the answer be improved? 
Doom 2016 Mods
But nevertheless I wouldn't count them anyway. You start this way and then everything which includes a little tweak here or there becomes a gameplay mod, even if it's Valiant, Strain, All Hell is Breaking Loose, or one of LWM's mods. I'm only counting those that actually changes aspects of the game.Wolf 3D TC doesn't really count, cause it is meant to be a Wolf 3D on GZDoom/Zandronum.I'm talking about the features you don't find in vanilla Doom (no not jumping crouching etc.
Doom Aeons Of Death
That'sfrom the sourceport).I mean like reloading from Brutal Doom, pressing 'use' on 'stunned' enemies in H-Doom, etc.Adding new enemies, weapons, huds, sprites, textures, music and maps don't count. Unless associated with a new gameplay feature.I added Demon Eclipse, since the wiki mentions it has a shotgun that that requires reloading (and I think the usage of an inventory too).And let's not bring the Extreme Weapons Pack. That was only a weapons mod (with some kinks added).