Just have him grow bitter and frustrated with his own stagnation in the cards until he finally snaps on someone, maybe The Miz or Jericho? Then he comes out and says he's been overlooked and under appreciated for too long and that, whether you agree or not, he's been the work horse around here for years! Yeah we all know how the song goes, but c'mon, if there's anyone who should turn and pull this card, it's Kofi.
Alternatively, turn him like they turned Jericho in late '01 culminating by backstabbing The Rock at Survivor Series. Kingston loses one too many #1 contender matches while the commentators put him over in the way JBL often does: "Kofi has all the talent but isn't showing us that killer instinct", and how he can't seem to win the big ones. He gets major heat by eventually losing it and costing someone else a big win, simply out of a new found stroke of jealousy. Additionally, he shouldn't address his distaste for the fans until he's done a couple of bad things and it's clear the people aren;t with him anymore. Then he can start going "...What? What are you looking at ME for!?" and have something to bitch about.
Yes, a lot of my solutions would involve Kofi snapping on someone and being unhappy with his own status rather than because of a beef with a babyface. But I think smaller, speedier heels work best as being cowardly, envious and slippery. And the bonus with Kofi is that he has something to whine incessantly about for heat (not getting his shot, the fans turned on him, etc).
As for his ring style, I thought of a neat "ultra" finisher for Kofi, like E&C's chair finisher, Orton's punt and Taker's chair guillotine. Where Kofi stomps a chair wrapped in someone's leg/arm from the top rope with big hang time. In matches he trash talks, slaps, uses his style for unique roll ups, or uses acrobatic ring escapes in tense moments to completely bring the match to a halt for heat.
With all that said, I agree with most others that a heel turn may not be the wisest move for Kingston. I mean I'd REALLY like to see it if it ever happened, but without knowing how capable Kofi is outside of his current schtick, the above is pretty much my best shot.