a prophecy said that azor ahai will be born from aerys 2 targaryen and rhaelee targaryen (dany's parents)
so it's either dany (their daughter) or jon (their grandson). in the books, rhaegar's other son is currently invading westeros so it's possible it could be him as well, but it seems the show completely cut him out and fused dany with rhaegar's other son : aegon 6.
Infact in the books, Dany is still in the East in Essos while Aegon 6 is literally causing chaos and war all over Westeros and Jon is busy getting stabbed by his nightwatch homies
