From what I've read, it's highly anticipated that Rhaegar actually loved Lyanna and she loved him. It's suggested that they had a betrothal, meaning if it was legitamised, Jon would be the rightful heir. Also a hint where Lyanna is whispering something to Ned on her deathbed that we do not know and begging her brother not to reveal Jon's true identity..
Remains to be seen but excited for the show to start. Each week waiting for the new episode is going to be painful.