Huh, funny that difficulty levels haven't really been talked about much! Very early on, while in Steam Early Access, we had an invisible difficulty that we would use to increase the challenge of the game the more you played and the higher you scored. It wasn't great though, the effect wasn't powerful enough to warrant the extra code complexity and it messed with the leaderboards.
Right now though we do see each game on its own self-contained difficulty curve. Also I prefer having a canonical experience that the game is created for and balanced around, extra difficulty settings do make both of those more difficult to achieve. Having said that, there is wisdom is providing different ways to engage with the same experience - we have many players who like Mini Metro just to zen out to, and others who deep-dive into it on a very mechanical level. So a lot to be gained by catering to both extremes and everything in between.