DOOM 3: BFG Edition
Enabling Nightmare Difficulty for all three Doom 3 BFG games
So you've finished Doom 3 BFG on Veteran and then Nightmare, only to find that Nightmare mode is still locked on the other two expansions? Here's the fix, and it shouldn't mess with steam achievements: Open the D3BFGConfig.cfg file in the
C:\Users\XXXXX\Saved Games\id Software\DOOM 3 BFG\base\
and edit the 3 'nightmare' lines with "1" instead of "0" and save.

In addition, I had to delete the \base\savegame\profile.bin file after making the above edit, and before launching the game, as Doom 3 seems to compare them for mismatches. Don't worry, the profile.bin file will be recreated automatically.

Launch the game and all 3 Nightmare modes should be available. This is also the method to make other changes, such as disabling the HUD for more immersion. Can BFG still use autoexec.cfg files? I don't know, haven't tried that!