The science and cultural districts will be built in most cities eventually, I'd imagine. The tech and civic trees are too important to ignore if you can help it. In the early game you might forego them, say, in a dedicated military city, but sooner or later you'll add one or both of them. This may be true of some other districts as well. It will be interesting to see how the governments and wonders will interact with the districts. I can imagine a situation where it makes sense to e.g. go all in on happiness wonders and policies, and get to ignore building the happiness district in alĺ cities, thus freeing up tiles for more farms or other districts. Etc etc... Can't wait for more detailed info on all these thingies!
