Pointless, Spain had the entire Iberia for 60 years after incorporating Portugal and they didn't rename their country just to idiotically fit geography

Maybe rather Celtica? Anyway, what provinces would it have? Ireland, Scotland, Cornwall, Wales, Brittany - very strange. It would never form without human and even human would have to develop in very weird way to get this. Not to mention the fact that this is complete and utter fantasy.
Tartar Union not only sounds lame but it is completely pointless - if particular Eurasian horde was conquering hella lot of land and other hordes, it wasn't renaming/reforming. It was simply Bukhara/Golden Horde/Kazan/whatever.
Anyway, do you really imagine legions of furious steppe warriors naming themselves Tartar Union? This is as much lame and anticlimatic as Hitler naming his country Democratic Republic of Germany 
Because we Slavs have an identity common as f***...
Two Sicillies*, also it is completely pointless given we have both Naples and Sicily and this country would require nothing but waiting till, for example, tech lvl 20.
'African Union' appearing in 18th century is as much climatic and making sense as Soviet Union.
Do you really wanna start a game as cool, exotic sounding tribe such as Huron and rename your country to super bland 'United Native Americans'? Especially since NATIVE AMERICAN is European term, 'America' is European term, and 'native' is European description? It's like creating formable European country which is called 'United Native Europeans'