When the big brother dies in Brothers: A Tale of Two Sons. The game is short, but does a great job of getting you emotionally attached to these brothers. Even through the controls, you control both brothers at once with each getting half of your controller. When he dies, it also essentially kills half of your controller. I found myself trying to move the brothers together as I have for the rest of the game.
Just play until you don’t feel like it anymore. If you need a milestone, clear a match/run/whatever on the highest difficulty. Or maybe every variety, so in Civ, a victory of each type. Or if you feel like going hard, find a rating you want to get and get there, like Celestial in Strive. I would say just play until you feel satisfied though. Don’t worry about what others think of your completion metric. None will satisfy everyone.