The problem with this suggestion is that France already starts the game over its diplo relation limit, I believe (five vassals and a guarantee, when its relation limit is 5). Going any higher and you wind up with a France that actually can't generate enough diplo points to annex its vassals in a reasonable amount of time while also actually playing the game.
Unless that's your aim, in which case I think your aim sucks.
Technically speaking France could just start with a modifier that allows it + to Dip Rel until it no longer has any of X vassals (X being the vassals it starts with), or until a certain date to avoid France sitting on one vassal or some such stupidness.
Frances vassals could have their own vassals.....
But then you'd have to start doing the same with Burgundy. And put Burgundy as a Vassal of France.
A neat suggestion is that you guys rework some of the governments. Feudal Monarchies have a minimum Autonomy for example. Duchies do not or get less.
the hordes have min Autonomy.
And it doesn't have to be increments of 25. It can be 10 or 15 or 30 or whatever.
....This might actually curb the super powered nations in 1444. They would have less income, less manpower, etc....
Although you'd just have to rework that + - autonomy thing in general. Just feels clunky with that 25 minimum to increase or decrease. And when you take an overseas province you can't increase or decrease at all - which is kinda important when you grab and small island province. You take some small island you could care less about any taxes or whatever the people are doing - all you want is a base for ship supply lines. So 100% Autonomy would be totally acceptable.
Italy:
Iberia:
Kingdom of Valencia (junior partner under Aragon)
Kingdom of Leon (junior partner under Castile) because: The union between León and Castile was never accepted by Leonese people. The Kingdom of León and the Kingdom of Castile kept different Parliaments, different flags, different coin and different laws until the Modern Era, when Spain, like other European states, centralized governmental power.
If the game were more like CKII then this would be possible since you have the Kingdom Laws to make it more united.
The Personal Union mechanic in EU4 just isn't designed to function like a proper PU.
paradox could always throw in the croes for these nations though. Every province in France is releasable so why not in Spain?
In regards to HRE nations:
You guys need to realize that most of the 'nations' were not nations. They were just Barons, Counts, Monastic lands, Cities, etc... who's feudal overlord was the Emperor. And since the Empire was dis-unififed they technically were independent.
Would be pretty ridiculous to have a section of the map with 0.001 base tax, and 0.001 manpower because its literally 50 people.
EU4 groups areas into a cohesive province for game mechanic purposes. It factors in economy, population, geography, etc...
Iceland is a great example. Population wise its almost nothing. But strategic and geographic factors gives it 2 provinces.