The biggest challenge for any Civ VI mod, in my opinion, will be dealing with the scale of play. Now that cities are 'unstacked' so that districts take up multiple hexes, how can we ensure that a map of Europe is both realistic in size and still playable?
In Civ IV, it was easy to hand-wave when designing a mod, to say that each tile represented a large region of the countryside (which is why a single city could have so many buildings and wonders within it). Now that cities are unstacked, you have to talk in terms of each tile representing, not even a single city, but a mere portion of that single city.
So how to address it?
One option would be to drastically expand the size of the map. This would obviously cause some problems -- specifically, game speed for slow or even moderately fast PCs, given how much map space we're talking about. While a bigger map is probably necessary, making it so big that the map can on its own compensate for the new scale is unrealistic. We need to make other changes to keep things reasonable.
Another change we can make is to limit the number of units on the map. That's a common issue I see in mega-map games of Civ V -- after a certain point, every tile is carpeted with units, which causes maps of all sizes (small or huge) to lose that tactical edge. Even better, this would match the historical reality, since armies in medieval Europe were really remarkably small compared to modern-day. Limiting military -- so that, for instance, you could only produce/support a certain number of pikemen from a single city's barracks, or only have a single knight for a city's castle -- would go a long way to keeping big maps playable. I'll expand this idea in a later post.
A final change we can make to partially 'restack' cities. Yeah, I know, districts are the centerpiece of Civ VI, so rolling them back sounds like mutiny. On the other hand, consider: the centerpiece of Civ V was the one-unit-per-tile rule, and we've already seen from the pre-release material how Civ VI has allowed units to be partially 'restacked.' It still doesn't allow unending Stacks of Doom, but there's quite a few screenshots showing different types of units (melee, ranged, support, civilian) and how many of them can move over the same space at the same time. 'Restacking' cities would be a similar change -- retain the principle, but modify how it's applied.
Specifically, here's how it'd work. I propose to treat every tile around a city as a district. Some districts would act as 'minor' districts -- these would include former improvements like farms or mining towns, or districts like harbors, that can be built anywhere in city limits, up to 3 tiles away from the center. The other districts, however, would act as 'major' districts -- these would be required for critical city infrastructure like markets and universities, and they would have to be built within a single hex or tile of the city center.
Now, instead of each city occupying an entire 37-tile region (all tiles at most three away from a center point), 30 of those tiles would represent the outskirts, the farms that feed the city, the trade-towns that dot the roads leading to and from it, the extended cultural and economic influence of the city in its region. Only the 7 tiles at the very center would represent the city itself.
This change would contribute significantly to the look of the map -- since cities would look like cities while countryside would stay rural. This change would also contribute to the proper spacing of the map, since each city would need quite a bit of space for its farms to support the inner districts. And finally this change would contribute significantly to keeping the map size reasonable, since each city would be of manageable dimension and could be positioned accordingly.