Disclaimer: I may get silly. Feel free to dispute my ideas in the comments or offer theories of your own.
Jon Snow: I hope he gets a happily-ever-after, but a bittersweet pyrrhic victory from George R. R. Martin wouldn’t surprise me. Best case scenario, he and Daenerys make a political marriage that ultimately brings peace and prosperity back to Westeros. And perhaps in time they’ll grow to love one another as much as Ned and Catelyn did. Worst case: he turns out to be Azor Ahai and dies saving the land from the White Walkers.
Sansa Stark: Marries Littlefinger, is widowed again (see below), and takes the Vale for her seat, extending Stark rule beyond the North.
Arya Stark: Reunites with Nymeria and cleans up the Riverlands with an assist from the Hound and the Brotherhood Without Banners. Rules in Rivverrun, extending Stark rule beyond the North.
Bran Stark: Physically dies but wargs into one of Daenerys’s dragons. Lives out the rest of his days as a magnificent flying beast.
Daenerys: Marries Jon and gets that happily-ever-after I’ve been brainwashed to expect from fictional ladies. Or dies along with her dragons to save Westeros. Or conquers Westeros, sets up a fledgling democratic state, and retires to the Summer Isles to conquer boogie boarding.
Tyrion: Revealed to be a Targaryen fathered by the Mad King. Prospers as either (1) the Hand of the Queen or (2) the Prime Minister of Dany’s new democratic state with Varys the Comptroller of Public Sentiment at his side.
Cersei: Strangled by Jaime.
Jaime: Dies in the arms of Brienne during the last battle.
Brienne: Dies gloriously during the last battle.
Samwell: Becomes a wizard, which automatically makes him Grand Maester. (After all, Pycelle is dead and the position has been posted.) First act as Grand Maester: changes the rules so maesters can marry. Second act: discovers something really helpful that contributes mightily to saving Westeros.
Littlefinger: Headfirst out the Moon Door. Remember his speech in “The Climb?” Time to fall.
Daario: Eaten by Drogon after betraying Dany.
The Hound: Knighted against his will by Arya for acts of heroism in the Riverlands. Also awarded a lifetime supply of chicken.
Theon and Yara: Down with the fleet to meet the Drowned God. The Iron Islands are given to Lyanna Mormont, extending that house’s influence far beyond Bear Island and teaching a fairly backward group about girl power, not to mention common decency.
Olenna Tyrell: Digs up some likely Tyrell bastard, legitimizes her, and plants her dynasty anew.
The Red Woman: Burned at the stake to forge/magically create Azor Ahai’s flaming sword Lightbringer.
Ellaria Sand: Makes Sand the official ruling house of Dorne. Seals the peace by marrying off the most annoying Sandsnake (a tough choice) to Robin Arryn, giving two loathsome creatures just what they deserve.
Davos: Created Lord of Dragonstone for his service to the realm.
Gendry: Still rowing.