Gilly slip legitimizes John Snow...annulment paves the way for legitimate union

In Season 7 Episode 5, Gilly practices reading while Sam Tarley scribes. She mentions several mundane facts recorded by a dead Maester..then just before being schussed by Sam, she asks, "What does annulment mean?"  After he explains, she states, "Says here [the High Septom] annulled a marriage from Rhaegar, and married him in secret to another women, in Dorne."

As we already know, RT + LS = JS, as verified by Bran seeing Lyanna Stark dying after childbirth, telling her brother Ned Stark to never reveal the secret, lest Robert Baratheon murder the child. Raised as Jon Snow, the bastard, we now know not only that he is half Stark, half Targaryen, but that he is now potentially not a bastard.

At common law, a child was considered to be illegitimate if the parents were not married to each other at the time of the child's birth even though the parents were married later. So by Rhaegar marrying Lyanna beofr Jon was born, it legitimizes Jon Snow, subject to the validity of Rhaegar's annulment.

The annulment given Rhaegar would be done subject to the law of the seven new gods.  The closest example we have is the doctrine of nullity granted by the Catholic Church.  Since Rhaegar marriage to Elia Martell was arranged, it is possible that the union did not, in fact, reach its fullness and become all that the Church believes a marriage to be, entitling Rhaegar to an annulment.  Even having children is, in itself, not a bar to an annulment.

So with Jon now a true Targaryen, he may be the legitimate heir to the iron throne as the last male Targaryen (book readers will disagree).   I am sure his Aunt would disagree.....