I'm always open to more formables when they make sense and have flavour, as
@moscal mentioned, often times you lose the unique things associated with your country before the formable, since you're an entirely new tag and usually these formables have nothing of use to them. Scandinavia is mentioned frequently, other examples would be Algiers, Westphalia, Germany, Italy and Egypt.
One of the issues though with formables is if they make sense or not, one of the big ones proposed, Austria-Hungary, makes no sense. Austria didn't from the just the Austrian Empire to the Austro-Hungarian Empire because they were nice, it was due to pressure from the old Kingdom of Hungary to have some sort of autonomy and run things their own way while still being in the empire, yes I know there was revolts and what not, but since I don't know the specific since frankly Austria-Hungary was a failed state so I don't research them fully. This would be like Great Britain forming England-Scotland, it's a downgrade. If Austria-Hungary was going to be added to the game, which I'm not
entirely against, it shouldn't be a "Yeah you upgraded!", it should be more of "You succumb to the demands, you lose prestige and stuff". Angevin Empire was never really a country, it was England, but they controlled French lands, something England tried to do before game start and has the opportunity of doing AND has their claim to France represented on their coat of arms at start of the game. Not only that but the Angevin Empire is a name we gave to it to describe the period of English rule in France, it was not an official name of country, and in French the name given is "Empire Plantagenêt", so the Plantagenet Empire, meaning it doesn't really work without a Plantagenet monarch, similar to the Ottomans. So once England gets the House of Tudor, or anyone not Plantagenet, Lancaster, York or even a somehow resurrected Angevin, it doesn't really work. \
Then we have tags that are divisive and probably won't be added and understandably so, rather it be because the time period they were around for or other issues such as name. Some examples would be tags, such as Czechia, Slovakia, Czechoslovakia, Slovenia, Yugoslavia, Zapadoslavia, Iberia, Lusitania, Portugalicia, Belgium, Celtica (Or whatever you want to call the Celtic Union country), Turkey, Belarus, Ukraine or Maghreb. (I'm giving European or nearby examples since that's what I'm most familiar with) These are all tags that have some justification, but also are held back. The ones I'm not totally opposed to are Czechia, Slovakia, Czechoslovakia, Zapadoslavia, Slovenia, Yugoslavia, Portugalicia, Turkey, Maghreb. For me the pros outweigh the cons for these formables, but the others not so much.
Formable tags that I would be in favour and seem to be generally accepted as possible tags would be Wallonia, United Kingdom (of Great Britain & Ireland), North German Confederation, South German Confederation, Denmark-Norway, Sweden-Norway, Lombardy, Piedmont, Poland-Lithuania-Ruthenia Commonwealth and Occitania to name a few, of course these are just some that I've seen and think could work, feel free to share your thoughts.