Pokemon Uranium. Poke-Chernobyl went boom and now there's a new pokemon type called nuclear. Nuclear variants of regular pokemon tend to go berserk and attack everything in sight, though the pokemon native to nuclear can be controlled.
Five Nights at Raiden's. Genshin Impact fangame where I guess Hoyo made a kids place for Genshin animatronics. But then the animatronics got corrupted somehow and now want you to die a violent death. It covers Genshin versions of FNAF 1-3.
Black Mesa because obviously.
The Jedi Masters mod for KOTOR 2. It's an alternate continuity that considers SWTOR to be non-canon. While too fangamey to be a true KOTOR 3, it puts the remaining plot threads from KOTOR 2 to rest.
I didn't get very far in it, but JPDE is an alternate continuity of RWBY, with the point of divergence being somewhere near the end of Season 1. The main character choosing to use a whip for combat is dumb as hell though.
SourceWorld. It's procedurally generated Half Life 2. Fun.
Escape: Yavin 4. It continues off from the Dark Side ending of Jedi Academy. The fun thing about this one is all the different kinds of lightsabers, and you can pick them off dead people to use them yourself! My favorite is the circle one that has blades coming out through the rim.
The Dark Alliance for Jedi Outcast and Academy. It's a three parter (The Mainframe, The Dark Alliance, The Dark Alliance II: Vengeance). The point of divergence seems to be between Outcast and Academy. Vader is back in action which kinda shocks the cast.
Galactic Legacy for Jedi Academy. In Fate of the Republic, if you play as Windu, he has better situational awareness than the movies this time. There are a few other interesting alternate timelines packed in it as well depending on who wins certain critical fights in the prequels.