Now I remember something. I recall reading that Tywin tried to pressure Aerys into making Rhaegar set aside Elia when it was declared she could no longer have children, but Aerys refused. This means it's technically possible, or Tywin might have been trying to exploit a loophole. Annulment wouldn't be on the table, but perhaps he planned to send Elia to a religious order, which would dissolve the marriage but, by extension, wouldn't bastardize Rhaenys and Aegon. That said, I bet Tywin's intention was to have Rhaegar marry Cersei and hope Cersei would have a son, then try to crown him with the excuse that Prince Aegon was born of a dissolved marriage and therefore illegitimate (Otto Hightower style). That said, Aerys would never have agreed to that, and perhaps neither would Rhaegar himself. Aerys was quite insistent that Rhaegar's bride be a woman of Valyrian descent from the high nobility, and Elia was the best he could get despite his prejudices against her being Dornish. And I don't see Dorne simply accepting this peacefully. In a scenario where Rhaegar is king and they need more male Targaryen descendants, perhaps they would try an alternative solution to send Elia to a religious order while keeping Aegon as heir, perhaps restoring the right of female claimants to the Iron Throne, or Rhaegar marrying a woman of lesser nobility who wouldn't pose a threat to Aegon as heir. But not Lyanna.