1) The most common thought if you watched the series...Sam is going to the devils side and Dean was bought back to kill Sam
2) The angel is Lucifer. Sam will have power to "wipe out" the demons from earth...hence, Dean who also believes Sam will move on to the dark side will have to kill him (but he won't)
3) One show finally has goes to the next level and says the devil and god are the same in a twisted way!
4) Something more bad ass (Wishful thinking)...or more lame.