In an Alternative Universe when everyone knew Jon was a Targaryen + the Targaryens still were sitting on the Iron Throne, who was in the first line to rule next? Jon или Daenerys?

Jon probably. Aerys was the king, Rhaegar was the crown prince so Rhaegar's son would come Далее in the line and since Aegon is dead, that puts Jon Далее in line for the throne. But Jon's claim could be weaker if Rhaegar and Lyanna weren't married.

So in short, Jon is the heir to the Iron трон if Rhaegar and Lyanna were married. If not, then Jon is still a bastard with a weak claim and Daenerys has a stronger claim.

Jon would be Далее in line if his parents did marry или if he was legitimised by the king. A legitimate child comes before a sibling - siblings are only heirs if someone has no children/only bastards.