The only problem with Karin is that they have practically no history together. She liked his Charka, but I don't believe he looked in her direction. Thats pretty much been the only interaction between them, on the same note he appears largely oblvious to Hinata, though thats probably due to the curcumstances they met in (when he got back from the battle with Pain, he was more distracted with Sasuke)
That being said, I would prefer it to be Hinata, but I believe like most manga of Narutos type, we will see absolutely zero romance in the pannels and everything will happen offscreen "I didn't know or speak to you much before, but now the screens are off we will flashback to 10 years afterwards, and you will be my wife." XD