Addicted Doom:
Story: Doom guy has become addicted to slaughtering demons so he got a nerd from Black Mesa to make a Multi Universe Time Travel device. (MUTT Device). Now Doom guy can kill demons from every universe in any time.
The idea: A big PK3 file that weapon, health, ,and armor upgrades, NG+, and new Challenges to unlock new Classes, weapons and Level Changes(Doom Gal Daisy, Duke Nukem, Caleb from blood, Maybe Civie11/Duke Nukem music, Ratchet And Clank 2 kinda synthase/midis, Blood Music, Rise of The Triad Music) and Challenge Options like "Unforgiven," (Your enemies are strong and your hate may not prosper, enemies are armored and have upgraded weapons), "Survivor"," (only 50 or less HP and you can't attack, RUN), and Forced Completion (Keys are locked behind Secrets, Items, and Kills, get em all to 100 to beat the game), last thing changed would be berserk, I'd change it to a weapon upgrade case.
Sound fun?