Here is idea comrade:
Each district should unlock a building path for your barony; so a certain district should, instead of adding new buildings, allow you to make a certain genre of buildings.
So a newly constructed Barony would have only one build option (i'd call it the "Castle District")
The "Castle district" would unlock the castle, walls, and fortifications (and maybe barracks). whenever you upgrade a castle wall you have an event that asks you what new district you want to build or expand. Once you choose which district, it will immediately be built/upgraded in your holding.
My idea for what the main districts should be is this:
Residential District: Creates revenue, unrest, and garrison size. Replaces Castle town. Very cheap and has many tiers. Second tier allows the Naval District to be built.
Military District: Creates upkeep and levies but decreases unrest. Allows military buildings as you upgrade it. Moderate price with a decent amount of tiers. Most important district for a capital.
Naval District (Coastal provinces only): Creates revenue, unrest, general opinion, boats, and increases technological advances. Makes herecies more likely. Allows Shipyards. But quite expensive and only has one tier.
Theological District (Christian, Muslim, and Jewish only): Creates piety, general opinion, cultural advances, and gives Religious-head opinion. Allows Chapels. Purchased with Piety.
Castle District (just to list its features): Creates Garrison, Levy, and upkeep. Allows for Castles; Castle Walls; Castle Fortifications; and Residential, Military, and Theological districts.

Vlad the Great!

I am looking to avoid tampering with Vanilla Assets because I want to maintain global compatibility, and replacing the common/buildings folder would reduce how compatible this mod is.

I do like your idea; I wanted to keep everything themed.

I will look into the idea of creating a mod that looks at more depth - and maybe re-name this mod to "Districts! Lite" or something.

There are somethings that I find odd, such as how the labour yard gives no benefit other than allowing you to get the third labour district. Also the same goes with the ballroom and the noble district.

Perhaps the benefits aren't showing, or aren't working, I will take a look at it.

Thank you for your feedback!

I like the possibility to be able to concentrate a lot of power in a single duchy,which in a longer game will have many highly developed holdings.So I would prefer not having any restrictions for lesser holdings (just let the ai have the same possibilities).

The buildings I would most love to see in an update would improve research in general in one of the 3 research areas each (for example through 3 new buildings) and/or in a very specfic area each through upgrades of existing buildings which already have a fitting theme,where such are available.

If it can be avoided to have buildings that take time and money to build generating collateral damages in one area or the other, I would prefer to avoid it.

If you want not to overload counties with additional buildings: how about some buildings depending on terrain-type (the idea is not new but still good imho) ?

I'd like to point out that as soon as you give away a castle or city to vassal all the districts vanish from the location, if you've already built them are they destroyed or simply hidden?

Economic buildings based on terrain and area. It would not have to be uber-complex like flogi's mod but a lvl 1 lumber mill in forest could give +1 tax up to the level of castle town you have. Farmlands and plains should have grain farms which work the same. Moutains should've have mines. Deserts should have oasis's and hills should have either more farms or quarrys. These would only be for capital holding of course. You could also make certain counties have unique economies for example the coast of gasgognce and bourdeaux should have wineries instead of famlands that give much more tax but they cost a lost to be upgraded. Certain centers of trade such as Seville, Paris, Cordoba, Rome, Rome, Constantinople should have a building that gives a tax bonusl (Merchant's wharfs for coastal, Stone bridges with tool booths for on a river and other things.