I think the issue with scenarios is that with the way modern civ plays, like there are too many elements to juggle. For example - what do you do about science/technology/civics for a scenario based on what would be basically one normal speed civ turn (if that makes sense). Besides having a game of "who can produce the most units", I don't feel like you can do too much else.
I remember one of my favourite scenarios from civ II, I think it was "La Belle Epoque". Strictly speaking, it refers to the period from 1880 up until WWI. If I were to design it for civ 6, I'd be starting somewhere after Napoleons defeat but before 1830, which is when "the great game" (British-Russian rivalry in central Asia) begins. It would be a very ambitious project to undertake....
For civ V I did have a go at a scenario that would have been 600 - 900 AD with the focus on the Byzantines, Persia, Arabia, China and Tibet. I wanted to put 9 wonders in the game, 8 unlocked by techs and 1 pre-built in a city state at game start. The idea would have been to control 7 wonders to win. But after a bunch of work, I still didn't know what to do about culture and the AI just could not play remotely well.......