My reasoning for Jericho (not Edge) turning heel
The reason why I felt that Jericho should go heel (instead of Edge), is because it then puts him at odds with Chris Benoit.
Jericho and Benoit had an AWESOME feud going 4 years ago, and I think they could do the same.
If Edge stays face, he can then feud with Christian. These guys already have the history, the background, etc. Furthermore - the WWE never really tapped in to their potential feud.
E&C together would be pretty cool to though.