I’m not sure what Colin was
for in the movie, other than to get Lisa acting dizzy and adorable. He needs a lot more development if he's going to stick around in the show. He’s wonderful, sure, but perhaps after a while all that perfection would start to grate on Lisa. (Remember Allison?) She’s competitive.
That isn’t to say that she needs someone who’s her inferior -- but I do think that Milhouse, already an interesting character, could grow into someone Lisa would feel equal to. Not necessarily mentally, just someone she could respect as a partner. He does have occasional moments of wisdom that are usually overshadowed by plain geekiness; a little like Lisa only without the development that comes from being a main character.
Of course, this is all hypothetical and I don’t see anything concrete happening in the show itself – either they’re still just kids, or it’s an ever-more-crazy flashforward. But my vote goes to Milhouse. He’s already a good character with potential, and he’s one of the easiest to sympathise with (any other female Milhouses out there?

). If the show was about Milhouse we’d be supposed to hope he gets together with the girl of his dreams.