Simply by what interests Martin, it makes sense to me that he'd want to "introduce" these characters who know, and yet don't fully know each other, who have so much baggage that can be unwound by their reunion (siblings who barely were, siblings no longer but children of warring families) there's so much he can explore there! All the trust that comes with family, but the lack of a relationship that established that specific trust, the pressures that might put them at odds (heirs with dueling claims), the things that would bind them together (the same dreams, the desire to rebuild Winterfell), and those things that make it difficult to find a status quo of what they are to each other/how to interact, Jon the bastard (outsider), Sansa the consummate lady (ultimate insider). And you’re right, the fact that Sansa now has experienced some time living as a bastard (and potentially patterned Alayne on Jon in some ways) adds depth there.
Their dynamic would allow Martin to answer the male/female inheritance issue, examine playing the political game but with principle, Jon's residual trauma from Cat, Sansa's with her dad...so many threads that could be woven into this dynamic in particular. I’m with you. It would a travesty not to play in this particular sandbox.
