My favorite open world game that ISN'T a fantasy rpg also happens to be the easiest "finish the game early" I've ever experienced.

Ghost Recon Breakpoint.

Basically you start the game in "realism" mode which turns loot levels off,. Then in the intro mission, when you see the bad guy kill a team member, you gain control of your character before he gets in a helicopter and leaves. Run forward with a scoped gun to shoot him in the head, or have something explosive ready to blow up the helicopter.

Congrats, you've killed the main bad guy of the game and the last level no longer has you fight him in some building.

You still have to do everything else in the game, but I managed to do that by accident my first play through. Put probably 150 hours in that game over the years and multiple playthroughs and still have yet to play with loot leveling on. I do sometimes let him live though just for funsies.