A few changes I'd like to see
1: Anti-snowballing or rubberbanding. In a lot of my games (especially science and cultural games) it's become abundantly clear that I've won by, like, the renaissance.. but in terms of real life time, the game is only 30% over. There's no chance of me losing or even getting a serious invasion, I just have to keep next turning until it's over.
2: Make t3 buildings better, becuase as is, there is very little reason to build them, except power plants and broadcast towers if you need a music slot.
3: Nerf wide. No, boosting tall or having the occaisonal tall specialist civ (like the maya) isn't enough. They need to nerf wide, becuase as is, it's always the optimal meta to conquer and city spam as much as possible and never have to bother actually building anything. And I hate that the pressure to do so is always there. This is the major reason I still go back to civ 5 sometimes.
4: City states should spawn with like, an archer and a wall. I'm tired of half of them dropping like flies before the medieval era.
5: Get rid of recruit partisans. Just scrap it altogether.
6: Diplomatic victory is still too slow, mostly becuase of the "this person loses DV" resolution. Yes, they lowered the loss recently, but it's still not enough. Almost every time I try to go for diplomatic victory, I end up acidentally winning by tourism or being forced to pivot to science instead because the rate of DV point accumulation is too slow.
7: Get rid of some of the more boring world congress proposals, especially "X person gets more grievances".
8: Trade deal spam. Especially demands from people who don't like you.
9: As far as I am aware, there isn't a way to sort all available trade routes from ALL cities so you can find the most lucrative one in your empire. You could do this in civ 5, but in civ 6 it seems like it only shows the trade routes from the cities you already have a trader at.