Westeros Ruler Tier and Government Type

It's not just a question of who I would have as King tier, it's also who would be duke instead of count and who would be count instead of Baron as well.

For starters, I would go with the kings; I would certainly have Hightowers and Royces as Kings with several duchies below them defunct or even existing and held by a vassal or themselves.

Yronwoods, Manderlys and Dustins are very likely candidates as well given the huge chunks of their kingdom they control, Osgreys at the height of their power too and perhaps Goldengroves after them as well.

It's also not who will be king tier but also where; I would split most if not all the kingdoms into several kingdoms, North and Reach especially would have many defunxt kingdoms.

One example of a split; Stormlands; Dornish Marches Cape Wrath (with all the isles), King's wood and Massey's Hook.

No? Not all the ducal houses. But certainly the ones who own too much land, especially compared to their respective regions.

Also bad example on your part; Dornish marches, even the part under SL is already broken into at least 2 duchies; Redwatch under Swanns and Dornish marches with no ruler. We know Caron and Dondarrion to be capable of Raising 4800 men together, though neither has a duchy. This is despite the average of these two, 2400, being more than Florent's 2000 even some hundred years back and being near the number of total men raised by Karstarks. Peakes, another Marcher house, who had 1000 men during Dance has their own duchy as well. So these two are examples of houses that need a duchy
You may ask how the king tier relates to this, it will allow to break down very large lordships into smaller duchies while still allowing the lord to control these lands without losing control over lands and it will also allow to hand out some dduchies where needed but can't be given right now.

The lords I have listed also stand apart from the rest with their power; During dance, Hightowers have raised 5000 men and this is without at least some of their bannermen, it may very well be they raised this number from Oldtown alone. Closest we get to such a number is Freys' 4000 in current time and if not mistaken, Freys have raised just 800 men back then.

Royces also stand apart, 6 Lord declarents are each capable of raising 1000 men, with a total of 20000, that is ~3350 men per lord on average, it is already close to the highest we see in the series, 4000 of the Freys, and we know Royces are much, much powerful than the rest and unlike many lords in the series, even has houses sworn to them disconnected from their land; Coldwaters. It may very well be houses besides Royce range just in the 2000-2500 area as we see with many other major houses, Karstarks, Florents, Caron, Dondarrion, possibly Reynes and even Tywin's own forces amounted to just 3500 men when marching on Reynes and Tarbecks

These kingdoms would be there to represent historical and Geographical regions besides administrative reasons as well.








Finally, since you are yourself suggesting to give the DS with the very small amount of land and meager military power compared to most lords, you should be able to see my point; DS needs a King tier so it can have duchy vassals beneath, so these duchy houses can both stand apart and have perhaps some counties beneath them where needed, such as Velaryons with their two towns and two castles.

I would also give each city and major town their own duchies with a county tier city/town (or perhaps several counties) seperate from the castle and governed by a command or something similar