Puzzle games seem like the perfect example of an entire genre that strives to get the difficulty level “just right.”

I think Stephen’s Sausage Roll might be my absolute favorite recent example of this. The Talos Principle and the Witness are also other great recent examples.

Also, to your last comment about adjusting difficulty in the UI: many, many single player action/adventure games have this. It’s been a commonplace thing for quite a while now. It also enables cool things like maximal-difficulty speed runs.

I recently had a great time watching someone speed run Metal Gear Solid 2 at its absolute highest difficulty level (that I don’t think the game even offers to players until the first play through is completed). I believe it was a recorded speed running-for-cancer event or something like that. I’ll have to try to find the YouTube video.

Actually, here it is: https://youtu.be/3DDMhcxUoAc. Obviously not something to watch if you ever want to play the game and haven’t yet. Another excellent one for MGS3: https://youtu.be/jUMDBOasRbA