Well, quite apart from Johnlock, which are my OTP, I think Lestrade and Molly have the best chance on the present showing.
Lestrade was visibly impressed when Molly took off her coat at the Christmas party in A Scandal in Belgravia, he specifically asked her if it was serious with Tom (and looked a bit sad when she said it was) in The Empty Hearse and they were sat together at the wedding in the Sign of Three, (she was sat between Tom and Lestrade at the reception), and Molly didn't look nearly as scathing about his 'murder' theory as she did about Tom's.

From a character point of view, much as I like Molly and love Sherlock, he'd be very hard to live with and so much of his behaviour makes her uncomfortable. Greg Lestrade in some ways is almost a 'gentler' version of Sherlock, he works to solve crime, he uses forensics, he's intelligent, tough and on the side of the angels but although he works within the rules, like Molly herself, he isn't afraid to bend them occasionally for the greater good. He's similar but less volatile than Sherlock and I think they'd be a good fit.