Is there any upside to Jon's heritage being revealed at this point?
So here Jon Snow is...his loyal followers from the North believing in him as the eldest living son of Ned Stark. The bastard status that has haunted him all his life is no longer relevant.
He has met and made an alliance, both politically and emotionally, with Daenerys Targaryen whom he respects and is willing to bend the knee to..She is much more interested in her claim to the throne than he ever was in ruling as King and he's chill with that. He's willing to support her and ally his forces with hers. They could marry and create the ultimate power couple even if they weren't smitten with one another.
But wait...Jon is not really Ned Starks son..He's a Targaren and the legal blood heir to the Iron throne (which he doesn't even want)..The chosen one..How will this be a positive reveal at this point? Will the North really be cool with all that? Will Daenery be willing to abandon what she felt was her destiny? And how romantic and allied will they feel with all these obstacles on top of her really being his Aunt ..I don't see Jon being cool with it..And how will Sansa feel?
And you know, of course, that once Jon knows the truth that there's no way he's going to lie about it even if he wanted to (which he probably would want to at this point...or rather not know at all).
This seems to me, at this time, a reveal that's only going to cause chaos at a time everyone needs to be coming together and I'm sure GOT will make the most mileage out of the difficulties involved in the big reveal.
So can there be a positive outcome from this revelation?