The most obvious new clue that Jon could be Azor Ahai likely jumped out to A Song of Ice and Fire book readers immediately.
If Rhaegar’s statement about a son named Aegon as the prince that was promised is accurate, then Jon is clearly going to seize his destiny as Azor Ahai.
In the Season 7 finale, we learned that Lyanna and Rhaegar had originally named their son Aegon Targaryen.
Besides, as the son of a Stark and a Targaryen, Jon is the product of ice and fire.