A)
The jonsa ship? Sansa stans who ship this seem to hope for Sansa to become Lady of Winterfell and be "compensated" for all her suffering in the bk or the show, and having Jon be married and/o emotionally devoted to her serves to make a moody pretty boy with skills be her commander, a la Daemon for Rhaenyra you might say. This is the priority from what I've seen so far.
It's another illicit or forbidden love as he's her brother and a bastard who her own mother who died for their older brother and them all disliked. It's also, I think, a fuck-you to GRRM's orig plan to have Jon fall for Arya not bc Arya was also his sister and even younger than Sansa, but primarily bec this, to them, places Arya as the "true" Lady of Winterfell.
I'd take a gander to say this is actually the issue and why some might say it's a Jonsa thing. Perhaps they make similar how Sansa's father has a hand in her encountering danger through her planned betrothal to Joffrey (Aerys continually endangering Rhaegar and his his family, Jon maybe in thei minds being the "cause" of Sansa/Rhaegar's family's destruction) or they posit an unwanted marriage parallel of Sansa-Tyrion and Elia-Rhaegar? There might be more, but I can't think of anything else?
B)
When you say "resemblance", do you mean to say Sara Snow x Jacaerys Velaryon as a true thing that happened, or this story of such meant to parallel the couple of Lyanna and Rhaegar? Perhaps it's the element of her also not having any hope to marry and--pursue a longer and enduring relationship due to the political circumstances--an unavailable noble man, like with Lyanna and Rhaegar? That Jace comes from a Targaryen royal lineage and Sara from a Stark one? Or is it that this ship is in close proximity to the Pact of Ice and Fire, which some people espouse had to do with the actual prophecy Aegon I is said to have had and maybe told to Torrhen Stark, the King who Knelt, that got him to surrender, and thus Creagn and Jace agreed to have their childrne marry--thus being some sort of precursor and affirmation of Lyanna-Rhaegar narratively & metaphorically "destined" to shape the history of their society throught their own singular action? That Torrhen might have already known, as others say? Which all, I believe , is an argument meant for Jon being the Prince that was promised?
Bc these latter two "ran off" bec their personalities and personal goals/values didn't jive with the "obligations" both had; those that were like stones set against their own and those around them (Lyanna with her marriage and the entire woman-must-obey thing/her being found out about being the Knight of the Laughing Tree and thus punished vs Rhaegar and the prophecy against the legacy of his house/wanting to build a different Westeros from his crazy father through said prophecy, to "fix" Westeros and/or the world in a way Dany is seemingly supposed to).
Whereas Sara Snow and Jace either don't have such obligations (Sara) or don't have such relationships or regards to their obligations (Jace). Unlike Lyanna Stark, Sara Snow--if she ever even existed. I don't think she ever did, Mushroom is Mushroom and was never there to witness what he said happened--was reported to be a bastard girl, who Cregan, her supposed brother, would never confront anyone about or for if she were to have pursued a relationship for precisely because she was a bastard girl who no one could leverage like Lyanna's father tried to when he betrothed her to Robert Baratheon. Rhaegar had to leave Elia with his father when said madman ordered Elia and their kids to return to KL/the Red Keep while Rhaegar was away; he also had several "engagements" that Jace did not have a like to before the war that Rhaegar was compelled to act on, presumably for the world and not just his own father, wife, etc. Jace was just living with Rhaenyra before the war, anticipating marrying a girl he had been close woth for ages since childhood, and he was barely in his own majority when he died not so late after the war broke out.
Jace's story, unlike Rhaegar, was that he was suspected as a bastard/not his father's son and that the Westerosi stigma against bastards and their character and ability to have loyalty is unsettled by his utter devotion to his family and mother's claim--in other words, he was all in both because of and despite the logic of those rumors, ironic bec of later when Rhaenyra's compelled and unethically chooses to arrest the dragonseeds when her council basically demanded that punishment--in a way, Jace's work was…troubled by this, even with Jeyne Arryn, the Manderlys, and Cregan Stark pulling through later but too late.
In other words, the stakes are too different and the characters are not so similar for Jace to act like Rhaegar or have the room & circumstance to do so. Whether you think Rhaegar was chaotic good, chaotic evil, or just pathetic. This is also not me sayign that Rhaegar wasn't devoted to his family/family's legacy, but that his motivations were layered and has a chance of being more about, again, saving the world. The Pact of Ice and Fire was bt Cregan and Jace also, again, was for their kids to marry, not Jace to marry a Stark bastard girl and disobey his mother at a time when she needed
What exactly are we meant to take away from Jace being with Sara Snow for the narrative apart and what does this contribute or explain exegetically or diegetically apart from it from the pattern of in-world speculation in F&B? Yes they share those that
I listed, but they hardly have similar narratives or "points"?
