Harry and Luna
Ron and Hermione
Ginny and Neville
Reasons:
Luna is the only person that Harry can sort of relate to in book 5. He's more open to Luna than Ron and Hermione. Yes she's a bit weird, but really not that weird. In the previous 4 books Harry has had 1 crush that didn't work out, and he shows no interest in the other girls. Luna makes a comment about standing under a mistletoe, but then screws it up by mentioning nargles.
Ron & Hermione is a couple everyone thinks will happen, but who's to say that it hasn't already started? They were both prefects together, which meant they spent a lot of xtra time together. At the end of the climax scene, Hermione kissed Ron instead of the usual hug. If they aren't considered together as of book 5, then I think its safe to say the foundation has been laid down on this relationship.
Ginny and Neville sounds to be the oddest, but possibly most likely relationship. I know everyone wants Harry to be with Ginny, but you don't go out with your best friend's sister. (that's like an unwritten rule in life) Who did Neville ask to the Yule ball? (ginny) and lastly, When Umbridge and the Inquisitorial Squad catches Harry and company talking to Sirius in OotP, Neville, who was not part of the distraction, is captured. Why? Because, even though his friends were being ambushed, he ran to the aid of only one person: Ginny.
Those are my thoughts and tho some might not see them as ideal, i would say they are definitly a possibilty.