Unpopular opinion: I think wide in civ 5 balances tall vs wide better than 6. Wide isn't optimal in 5, but it can work. Tall in 6 just sucks and 6 really pushes you to spam as many crappy cities as you can.
I don't think this opinion is unpopular, in fact I more than partially agree with it. As for tall in civ6 sucks, I agree and think there's a pretty obvious solution based on another problem that we already have with the game mechanics: 2nd and 3rd (and I propose additionally 4th) tier buildings. The first tier buildings are cheap and cost effective. They double the GPP output of that district (1 becomes 2,) And if map-luck doesn't give you a premium spot for good adjacency bonus to the district, the building provides at least a solid flat per turn generation (looking at you, theater squares.) Some also unlock additional powerful game elements, most notably first tier CH and HB buildings giving an additional TR slot. And yes, while I'm generalizing "districts," I'm talking mostly about the campus, CH, and HB. 2nd, and especially 3rd tier buildings, come later (and respectively MUCH later) so there's fewer turns for returns on the investment of having them, and they don't add much of a bonus other than a flat +5/+7 to the yield. Third tier buildings aren't even supplemented with CS or GP bonuses. And they're so expensive with less time to get returns on them. And there's less of a percentage increase in the GPP generation (where a library in a campus increases from 1 to 2, doubling it, a university brings it from 2 to 3, so only a 50% increase.) So as it stands, the cost of making 2nd and 3rd tier buildings isn't as good of an investment as the cost of settling another city and getting a campus and library in it.
Instead, I propose that the 2nd tier building should be something that greatly increases the city's capacity for population, something along the lines of needing one amenity for 3 pop instead of 2 for one type of district's 2nd tier building (campus or theater square, but mutually exclusive) while a different district's 2nd tier building (CH or HB or IZ, but again mutually exclusive) allow 1 housing to support 1.5 citizens. These second tier buildings should remain as expensive as they are, so that a tall narrow empire can justify the expense of the three or four of them needed to have one in every city and really cash in on how much taller the cities can get, but a wide and short empire can't justify having them in all cities. Additionally, perhaps the 2nd tier building provides a slight modifier to the adjacency bonus of the district so that the wider empire may want to invest in the one or two of them that have that optimal placement and disregard them elsewhere.
And 3rd tier buildings should give a per-citizen (or 1 for every 2, or 1 for every 3, whatever works best for balance) bonus to the district's primary yield.This could really bring the balance of tall vs. wide.
Finally, a 4th tier building provides a percentage modifier to the city's output of that respective yield, including tile bonuses, the adjacency bonus, the flat bonus of the 1st tier building. the enhancement on the adjacency bonus from 2nd tier building, and the per-citizen bonus of the 3rd tier building.
If this were the case, you have the option of either having a tall/narrow empire that's easily defensible that turtles until it can unlock and cash in in a huge way on having very few but very advanced districts, or you could have a wider and shorter empire that can't benefit from these high per-pop and percentage bonuses, but doesn't need to due to the vast accumulation of nickel-and-diming from egregious amounts of lowly developed districts. Now you can go tall, or you could go wide, or as we at the forums do, find the ultimate sweet spot inbetween.