Personally, and also speaking on behalf of most the rest of the player base - we’d prefer if it were ALL Dragonflights, not just one.
I made my own thread advocating for it already – The 'Leak': New Race - Dragonkin!
As for -
Wrathion didn’t mature so quickly - at least not as a Dragon: You see throughout the expansions when he goes into Dragon form he’s just a whelp and gradually becomes bigger.
Being Dragons, some are born with having immense innate power which in turn also matures their minds - I think Wrathion casually dropped a line towards how he’s mentally mature in the rogue questline in cata but yeah. Also, the Black Dragons are slippery little wyrms – Their numbers aren’t disposed of as easily as one may believe.
This also being stated - Wrathion could have been collecting eggs since his hatching, or gathering uncorrupted minds of his flight - such as Ebonhorn.
As for all those suggesting ‘The leak is bs (Bologna Silliness) / lies’ -
Although that’s likely - I recall many of you suggesting the same about the Legion leak and concocting massive essays to why those believed it were fools & why its wrong (Tad awkward for them after it was officially announced, eh?).
A lot of people believe the next expansion to be ‘Light VS Void’. Though having a more grounded expansion after Shadowlands makes sense from a gameplay perspective, however I kind of feel it may be something of both - starting off simple, building up momentum and snowball into that subject.
Additionally, if they have all the Dragonflights as playable not just one - then they could have a Dragonsworn class too -
Having both race and class gives it relatively good justification – Now we have the Dragon Race of all the prime flights (Black, Blue, Bronze, Red & Green) and a Dragonsworn class. As you know, currently Dragonsworn are more of a sect than a class (Like in the game you even see ‘Dragonsworn’ that are other classes such as priests, mages etc) but with good world building, I’m sure it could be forgiven for change —
You could justify it via those among their kind that instead of choosing the path of another class - sought to refine their draconic skills, and find a semblance of its power whilst maintaining their humanoid forms to amplify said-powers. That’d be a good way from a lore perspective on bringing it to life.
Perhaps have the other flights sharing the ways of their flight to you - then going on a quest to slay a ‘Bad Dragon’ / Traitor of their flight & absorb that essence of their power to make it possible (Like the Dragonborn essentially).