Well, R+L=J. It’s pretty clear. Timeline-wise, we learned a lot from WOIAF.
This makes me wonder why they didn’t try to pass Jon off as the child Ashara lost and keep him in Dorne, unless Ned promised Lyanna he’d raise him/the Daynes didn’t want a Targ child around.
I think it’s a combination of a couple things:
1. His promise seems to have been, per Bran’s vision, to raise Jon as his own son.
2. The Daynes are a bit close to ToJ, Ned visiting them is public knowledge, it’s a bit shaky as far as alibis go.
3. It’s quite possible Ashara’s stillbirth is too publicly known to pass off Jon, especially if the timing is off by several months.