This is most AC games for me but Black Flag is the best example. Loved that game and really enjoyed just doing pirate stuff. I did progress in the main story but not sure how close I was to finishing. If I ever play it again I will do the exact same… everything outside of the main story was the best part!
Skyrim.
Once I get the first shout and visit the dudes on the mountain I Fuck off and go do my own thing.
Have never beat the main story.