Okay gentlemen, deep and dramatic discussions in this thread have created huge mess so again I decided to list all plausible debated ideas for formable nations.
Unofficial rules for Paradox - made Formable Nations in EU4 1.7.3:
- we can already form countries which historically appeared many decades after 1821
- we can already form countries which are not 100% realistic, such as formable Italy with Napoleonic flag and non-Napoleonic borders
- we can already form overlapping countries: Sardinia-Piedmont vs Italy
- we can already form countries which have never existed in the exact form shown in EU4: Hindustan, Malaya, Jerusalem, Holy Roman Empire
- we can already form countries which have extremely easy requirements - Bukhara, Kurland
- we can already form 'unrealistically stable cultural unions' - Scandinavia (this is my answer on the critic of concept of Yugoslavia)
I guess neither Paradox nor majority of player cry over these minor (?) inaccuracies 
Thus I am not sure if attacking new ideas for Formable Nations based on these points is validate.
Other Unofficial Rules for Already Existing Formable Nations:
- formable countries can't be completely fantasy/ahistorical/unrealistic, they need to have either historical background or cultural background (Malaya, Hindustan, HRE) - I mean, they could appear in this period
- formable countries shouldn't be based on short - living ephemerical states (example: United States of Belgium from 1790, existing 325 days) - they should be based on something meaningful
- they shouldn't be extremely anachronic such as, for example, Sparta (
) or Carthago - the most anachronic Formable Country in EU4 is currently Jerusalem but it could arguably appear again in EU4 period if some lunatic Catholic country resurrected Crusades.
- they need to provide flavor and fun to the game!
Sooo I have written below the most plausible/least criticized (or controversial) suggestions from this entire thread which arguably fit into the Ten Rules of Formable Nations.
1)
Belgium
2)
Austria - Hungary -
controversial as hell 
3)
Restored Mongol Empire - hey, we can already restore Byzantine Empire right?
4)
Arabia
5)
The Caliphate - the result of Unify Islam decision?
6)
Restored Hungarian Kingdom - possible only for Transylvania (it tried to restore Hungary for long time after it has been annihilated). Hungarian NIs.
7)
Anglo - French Union - as the result of clear victory in Hundred Years War?
8)
Restored Majapahit Empire - alternatively 'Indonesia' or 'Nusantara', overlapping with Malaya in the same way Italy overlap with Sardinia-Piedmont. Unique National Ideas (Majapahit in 1444 doesn't have them)
9)
Kingdom of the Two Sicilies -
controversial as hell 
10)
Yuan - the result of Mongolian hordes conquering China
11)
United Kingdom - England, Wales, Scotland and Ireland as opposed to Great Britain (England, Wales, Scotland)
12)
Vietnam - Dai Viet + Champa + Prey Nokor province of Khmer -> it is what happened historically in this period.
13)
Durrani Empire - Empire formable by Afghanistan: essentially Afghanistan+ Pakistan + Nishapur. Unique NIs.
14)
Great Mandala - modern: Burma, Thailand, Laos, Cambodia, Vietnam -> of course this one is ahistorical but similarly to Hindustan (?) and Malaya. Unique National Ideas.
15)
Restored Khmer Empire - Khmer + Champa + Laos + Thailand. Unique NIs.
16)
Maghreb - Morocco + Algier + Tunisia; based on historical empires of Almoravids/Moors and common Berber culture. Berber National Ideas.
17)
Kurdistan - possible to do in AOW map with expanded Caucasus, would require Curdish cultural tag, probably possible to form only by rebellion. Unique National Ideas.
18)
Axum Empire - Ethiopia + Adal + Eritrea. Possible to form by Ethiopia and has its ideas.
19)
Somalia - Ajuraan + Adal. Unique National Ideas.
20)
Missisipi - Native American Empire constisting of provinces of 9 tribes close to Missisipi River: Cree, Chocksaw, Cherokee, Osage, Caddo, Pawnee, Susquehannock, Miami. Capital in the legendary city of Cahokia.
21)
Turkey - possible to form by Beyliks if Ottoman Empire falls (???) or if Ottoman Empire loses Constantinople (???)
22)
Siam - Late game formable for Thai countries, insta-change to Absolute Monarchy and bonus towards Western tech. National Ideas of Ayyuthaia
23)
Myanmar - cultural union of six AOW Burmese states. Has to be called Myanmar - Burma is purely European term

Has unique National Ideas
24)
Srivijaya - Must have the Srivijaya dynasty in power (only Malacca still has this in 1444), requires Sumatra, Peninsular Malaysia and Western Java. Overlapping with Malaya and Majapahit Empire. Buddhist conversion.
25)
Punjab - formable by countries with Panjabi culture.
26)
Yugoslavia - Croatia, Bosnia, Serbia, Montenegro. Not less plausible than Hindustan. If formed by Serbia carries uses Serbian NIs, in other case has unique National Ideas.
27)
Australia - colonial state
28)
Philippines - colonial state, unique National Ideas
29)
Boers - colonial state, unique National Ideas
30)
California - colonial state
31)
Kingdom of Leon - if Castille goes wrong
32)
Al - Andalus - if Castille goes EXTREMELY wrong

Possible to form by Granada, Fez, Morocco or Algiers, switches on Granada NIs.
33)
Samarkanda - ahistorical but not impossible to form in this historical period. Unique National Ideas (super trade).
OK. Now you can debate on these