Not Soulmates
I'd really love to see Blaine realize that he and Karofsky are not even close to soulmates. Like if they were hanging out in their apartment on a weekend, with Dave watching a basketball game, and Blaine breaks into a song (like Whoomp There It is), and dances around Dave trying to get him to join in or dance with him, and he looks at Blaine like it's too weird, and Blaine slowly acts deflated. He'd have one of those 'gee, this would have been way more fun with Kurt' moments and just go back to making his Soda Stream drink.
Same with Kurt... he and Walter would be at the Lima Bean, and a bunch of New Directions would pop in, start into a song (like Taylor Swift's Shake It Off) Kurt would join in, and Walter would try, and then end up throwing his back out or something, and Kurt would have one of those 'this would have been way more fun with Blaine' moments.