I think this may be the first time I post on these forums (or if I already did it was so long ago I don't remember), but I decided to log in to give my opinion on this matter. Having played the beta (a little bit), I must say I vastly prefer the old system of districts and buildings only, which I think could be improved instead of discarted. I don't believe Paradox will abandon the new system, but I would like, nevertheless, to give my opinion on the changes the old system could have received.
About the districts, I believe (as others here) that creating more types would have been enough, instead of adding zones. However, it is on the buildings I would like to focus, as I like the immersion they provide to the game. Another person here suggested that buildings could have multiple upgrade paths, like research buildings had in the past, and I agree. Precinct Houses could be upgraded to the current Hall of Judgement, that further decreases crime, or to a more civil-focused court system, that reduces less crime but increases trade generation. Fortresses could be upgraded to a more navy-focused buiding, that further increases naval capacity, or to a more defensive version that makes the planet even more difficult to be invaded. The housing buildings could be upgraded to focus on more housing or more amenities (popular housing vs luxury housing). Even buidings like Psi Corps could follow this rule, like being able to be upgraded to a psionic monastery focused on unity or to a more repressive version focused on crime and stability.
Ethics could also influence this system. Holo-Theaters could, for example, be either upgraded to the more generic Hyper-Entertainment Forums or to an ethos version, like Gladitorial Arenas for militaristic empires or Elysium Parks for pacifistic ones, which could offer less amenities in exchange for increasing ethics attraction. I can think of many other exemples that could make planetary management more tailored to each type of empire, which, in my opinion, would increase immersion.
I don't think, however, that all buidings should be planet unique; instead, the upgrade paths that should be unique. Using tha reseach buildings as an example, I could have all specialized labs (society, physics and engineering) in a single planet, but, if I decided to build a fourth research building, I would not be able to upgrade it beyond the first level, discoraging its construction without actually prohibiting it.
Other buildings could even be added that serve to synergize with others, like an university buildings chain that increases overall planetary productivity on the first level and can be specialized to further increase engineering, physics or society output from scientists. In this case, if I wanted to specialize a planet on all science, instead of constructing a bunch of the same buildings, I could build a lab and a university for each specialization, consuming 6 buildings slots in total. Or I could focus on a single reseach path and use only 2 building slots.
Anyway, those are just some ideas I have. I don't know if anyone will agree with them, but I wanted to put them here anyway.