If Kofi turns heel, it's hard to say how WWE will go with him. We've seen Kofi in the midst of significant pushes in the past only for Vince to lose interest somewhere in the middle of things. As a result, it's hard to really get a firm grip on where WWE would come from. After all, Kofi has spent his entire career in WWE as, basically, just a generally happy & energetic babyface.
At this point in time, Kofi's biggest ally is R-Truth. The age old means of turning someone heel is to have them betray their closest friends and stab them in the back. The downside to this is that, at least for me personally, I have no desire to see Kofi Kingston waste his time feuding with R-Truth. I'm not saying that he should immediately be bumped up and going after a World Champion, but a feud with Truth isn't going to elevate Kofi all that much.
To elevate Kofi, a good feud to put him in would be against Chris Jericho. Jericho excels and putting over other wrestlers while still looking like a million bucks himself. Kofi is someone whose shown ability on the mic but if he still needs work, which I imagine he does, then Jericho can easily carry his own weight & much of Kofi's during promos. As for the matches, I think it's safe to say that Kofi vs. Jericho as part of a real feud is a potential show stealer.
Another potential path would be to reignite his feud with The Miz. They had some great matches on Main Event for the IC title. At the same time, Miz is someone that crowds love & hate. Depending on where they go at it, Kofi would be massively cheered as a face while Miz was booed. Again, part of the trouble with smarky crowds.
Maybe Kofi could feud with Antonio Cesaro for a bit, take the US title from him and then ultimately go heel. Cesaro has worked his ass off for WWE but I have a feeling that Vince has ultimately lost interest in him.
Other potential feuds leading to a heel turn, or coming about as a result of one, could be Orton or Sheamus, possibly even Triple H. The last one is the most unlikely, but would certainly be a show of proof that WWE is seriously behind Kofi as a heel.