/vr/ - Retro Games

Memes aside, my favourite vanilla-esque mod for D3 is cstDoom3. It's got features like an aspect-corrected HUD, the removal of screen shake from damage, and an option to turn off muzzle smoke particles. It's also got some gameplay-related toggles like a headlamp that actually sticks to the crosshair and the ability to show locker codes on the HUD whenever you get up to one.
It's a modular experience which I'd 100% recommend for any first playthrough of the game.