I think Chip Kelly as an offensive coordinator would be a great move. But since he has a good job now and is being talked about as an NFL head coach, I don't see him taking that position, especially not with the Bills.
Living in Eugene Oregon I have seen first hand what this guy does for his football team. His practices are like no other. Its all about conditioning and execution. Look at how the ducks play every week. They start off slow and eventually destroy their opponent with a higher level of physical play.
Also, this guy is a winner. Plain and simple. The attitude of Oregon players is of no compromise. Just listening to the guy talk in his interviews makes me think that he expects nothing less of everyone around him.
Chip uses the spread offense which would totally incorporate into the current Bills offense. If you don't want a complete restructure you have to bring someone in that is familiar with the current offensive scheme. This guy could bring this team to the next level.
That said, I'm with SJ. Jumping from college to NFL head coach with no previous NFL experience is just too much.