There are plenty of older games where running some kind of widescreen or compatibility mod to get it working on newer hardware is the only way to get it running well, but that's kind of a boring techie answer.

In the spirit of the question, I'd say Fallout 4. The base game has a story I don't care about, factions that make no sense, and very little in the feeling of actual threat. I usually run between 200 and 300 mods to turn it into a truly post apocalyptic hellscape with functional radiation storms, low visibility in dust storms, darker night, less HUD clutter, more ghouls, proper flashlights, retuned weapon damage so things aren't so spongy, lots of new gear and weapons (I add in real guns but try to be tasteful and not add too much super modern tacticool stuff, but more cold war and vintage guns), backpacks, re-dress the Minutemen so they don't look like 1776 LARPers. Add tons of new sidequests and stories to find. The world becomes, this amazing, terrifying place to just explore and forget about the main plot. Returning home to my concrete walled safe settlement and my personal bunker living space really feels earned after exploring.