Post-1980's - WWE has done a lame job with heels. Fair enough, it was a different era where kayfabe was alive and well and the likes of Roddy Piper were villified to the point that people would try and attack them in public. The point is, the WWE doesn't seem to have evolved so much with their mentality in creating heel characters.
Many of their heel characters follow a similar pattern modeled after the 1980's heels. The trouble is that in an era where kayfabe is redundant, the classic heel is no longer effective. Here's the thing, John Cena is perhaps the most over heel that the company has despite the fact he is playing a classic babyface. I feel that wrestling should evolve to reflect this and characters should be based around elements that one demographic loves and another demographic hate.
It may be complicated to pull off, given that the "classic heel" of wrestling will be phased out, but this is WWE and they are adamant that they are more than just wrestling - so I'd like to see them try this method of storytelling. They've basically opened it up for John Cena since he's essentially doing it already. Furthermore, have you seen the psychology that Cena displays when the crowd is clearly against him? It's absolutely magnificient and a real sign that Cena is more than skilled to play a duel role.