It’s present day, Westeros (imagine: present day UK-ish but nicer) is under a Monarchy where present Jon is heir to the throne (The Targaryen Dynasty).
Sansa is the daughter of the Duke of Edinburgh, Prince of Scotland (in my head: Scotland is still under UK but the old, the Scottish royal family get to keep their “titles” & land but hold no true power or are not in line for the British Throne. The Starks are the Royal Scottish Family and hold all of Scotland) Their relationship with the Targs are courteous but strained, as the Starks are descendents of the First Men who dwelled on the Land, while the Targaryens are a family of foreigners who conquered the Land (and are also inbreeders. imagine: current house of Windsor formerly house of Saxe Coburg and Gotha - a predominantly German family who’s cousins married each other so on and so forth).
Jon and Sansa meet and fall in love - here’s the twist: They also start to remember their past lives (a gift and a curse given to chosen ones who descend from the First Men. Jon’s mother is still Lyanna of the North / Scotland - but is not Ned’s sister...she is instead, a beloved cousin from a smaller branch of the Stark family - still a Stark, but the only woman Stark at the time for Rhaegar marry so as to reduce conflict with Scotland. Jon and Sansa start remembering Old Westeros where Sansa ruled as QITN, Jon killed Dany and became King, and fought everyone so he could marry his true love: Sansa. Sansa becomes Queen of Six Kingdoms - a sacrifice she makes for her love. Bran & Arya rule the North.
Their current life will be somewhat a mirror of their past life: Jon struggling with the inner wreckage of his family (inbreeding has made his family weak in constitution & in mind, his half brother Aegon died early of hemophilia & half sister unhappily married to a German cousin, a Grand Duke of an abolished monarchy but still a powerful man. Jon is half Stark, so he is not addled by the same sickness. He is the only hope of the Monarchy). Sansa still a wonderful politician (she is her father’s best adviser). The Starks and Targs don’t have the best relationship, but the Starks value family and the betterment of Scotland above all so they don’t openly cause any uproar against the Targs to prevent war - but it is common knowledge that they don’t like them conquerers.
Sansa while she falls in love with Jon (and she remembers their past life, their “Golden Age” of reign as King and Queen of Westeros 500 or so years before, their love story that sparked books and plays and legend) is reluctant to be under the pressure of a throne, surrounded by Jon’s poisonous and unhealthy family. Jon tries his best to win her over, because he knows Sansa is his past , present and future, and as such she is the hope of the Monarchy’s future - their children would be the best rulers, and the future of house Targaryen will finally be free of too close inbreeding, incest, madness and corruption. If Jon & Sansa don’t become King & Queen in the future, the people will finally revolt.
The Press will comment on how much present Jonsa looked like the legendary Rulers King Jon Aegon the Reluctant, the Good King, The Dragonwolf and Good Queen Sansa, The Red Wolf, The Most Beloved. The press and the public go crazy over present Jonsa (imagine: obsession with Will&Kate / Harry&Meghan) and while this improves the Monarchy’s standing with the public, in private this causes a lot of tension, as Sansa does not see any advantage of becoming a queen to a stolen, broken-down throne, as much as she loves Jon. Plus The Targs hate that a Stark is gaining much popularity and support.
LOL i have so much time on my hands, but alas i lack the skill to write a fanfic of this magnitude. If anybody would love to take this idea and do their spin on it, PLEASE GO AHEAD. I am living for Monarchy & Fanfics ... and of course, JONSA.
My version of Jon is a bit Dark, in a way that he is not a simpering, pouty fool like S8 D&D made him out to be, but rather like Battle of the Bastards Jon where he is ruthless when he fights for Sansa.