Well there are many nations that shouldn't be formable/restorable in EU time frame. Lets start with Mongol empire or Ilkhanate or even Lotharingia. All these countries seased to exist long ago and you can still restore them. EU time frame also touches Victorian era, where countries like Germany or Italy are formed. Just give them a little push in ideas, let them be formable tag at tech 20, make Austrian and Hungarian main cultures. Ultimatly it should be the player to decide whether to form it or not.
You should feel differences. Something other is RE-BUILD country, when you base on old traditions, and something other is CREATE country on new traditions, traditions that do not exist and and existence of thoses cannot take place in thoses times.
The sentiments towards the Mongol empire continued. Many great chiefs want recreat horse empire, the last was propably Timur the Lame. Similar idea of Roman Empire functioned in the social consciousness.
Concept of united of Germany or united of Italy had place in timeline of EU4. "Prince" was form of instruction for the future unifier of Italy. August II Starke want use polish trone, create second duchy for Wettings in Livonia etc, to have gold and power to unity Germany without Habsburg supremacy. And this is only small things of whole history of Italy of Germany about ideas of unification.
These ideas existed. The Austro-Hungarian idea is not. KUK was only pragmatic effect of a failures of austrian monarchy, not as effect of ideas, concepts or something other.
And methodology "anybody can be created and player should decide" haven't sense. So why not form CCCP? Or Volksstaat Bayern? Why not Polish-Bohemian Commonwealth? Why not Makhnovia? For thoses things, who is in deep conflict with alt-historical visions of Pdx and main core of players, we have mods.