With buildings not really cluttered anymore, being divided between the districts and city center, I do think buildings that needed specific tile / resources will either not have those requirements anymore, or will become tile improvement instead. Because you already need to take in consideration positioning for the bonus the districts get, I think we won't also have to take account if there is an specific resource of tile type nearby.
So for example, I think that the Stables,
that was mentioned to be in the military encampment, won't have requirement anymore. While, I would expect buildings like Solar plant, Nuclear plant, Hydro plant, Windmill, to be tile improvements instead.
On the list of districts, besides the 8 pretty much confirmed ones (science, faith, military, harbor, happiness, culture, production, gold) my guess for the remaining (many already mentioned by others here):
*health district (related with food with hospital and medical lab, for instance)
*tourism district (with buildings that affects tourism like airport and hotel)
*law district (courthouse, constabulary, Police station)
For the last I'm thinking of either an defense or garden (great person generation) district.
When it comes to defense, walls and castle are apparently already on the city center (with walls also on the military encampment). But I don't think the later defense buildings would go in the city center as it would make it too cluttered (or a new one may just overwrite an old one). They may also be turned into improvement instead (but it could be overkill to have buildings like that, stronger as the time passes, all over tiles in your culture that isn't).
One problem with a defense district, is that the military encampment already can do that to some extent while also likely giving bonuses to military units. Although a defense district may be really tough, maybe tougher than the city center and could have some other bonus like also needing to be taken to conquer a city (the city administration transferring to it in case the city center falls, which would make sense as a very strong defended position). Which could result in a situation where you can't do anything on the city as it's occupied, but neither the enemy and it's still part of your territory, so while the enemies need to fight it out with a strong defense district, you may be able to get reinforcements and stop them. Or they could take the defense district first, but a mechanic like I mentioned would avoid they just ignoring the district and going for the city center gaining control of it.
