Start with Klaine but KurtBastian endgame. Set end of season 2 and in between of season 3. Clearly an AU, so not much following about the Canon timeline.

Kurt and Blaine go to see a medium to know what their couple would become with their long-distance relationship. In lieu to look in her crystal ball, the seer show them an another level of voyant. She just switch two entities from different timeline and let them deal. So, a ten years later Kurt Hummel juste appears in front of a gobsmacked teenage Blaine. But only for five minutes. He learns in that short time his boyfriend got an internship at Vogue dot com, that he attends NYADA and lives in New York... Without him. Because, in the future, Klaine isn't anymore and Kurt lives better after Blaine hurt him. The latter couldn't believe it, it can't be possible, Kurt and him are just mean to be with each other, not any other way. Yet, Future!Kurt explains him that their love may be passionnel, it didn't last in long-term even if they try really hard, in the end, they would just hurt each other. Blaine refuses to face the reality and promise that he is going to change the future and make them the perfect couple. Grow-up Kurt just smiles sadly at him.

This version of Kurt is what Kurt would have become if he end up marrying Blaine Anderson at their Brittana wedding, because their relationship is just an on and off again that breaks their heart, they divorce after two years married.

And Kurt end finding true love with someone else, that young Kurt barely meet during those five minutes.

What he knows is that he got to banter each other a lot and they are really domestic with snarky come back. Kurt has to admit he quite likes this future but is kind of confuse because it's definitely not with Blaine while he just loves him so much. For now.

That's why at beginning of the new school year, Kurt asks Blaine to transfer to McKinley. He accepts and turns out, it's a really bad idea as Blaine tried to hold him in Ohio rather than let him out. Hence ending the relationship way before the actual timeline. So snarky Sebastian met a single Blaine and got bored quickly and set his eyes on Kurt, trying to disturb him during the years and glee competition. They end up finding each other at NY with Chandler as wingman.