You bring up a good point about Sargeras being seen when Medivh was killed. Evelyn described the initial entry into Aegwynn as placing a part of himself in her. This is an area where Blizzard really has the ultimate freedom to do anything they want. It is fantasy and they only get into problems if they contradict themselves but not in how they explain things because ultimately it comes down to 'Oh, it's magic.'

Regarding the corrupted champion theme getting boring, I disagree. There is always going to be that next villian. How varied can you get into why they are villians? Ultimately, at some point, any true villian has gone off his/her rocker and is now doing bad things. There are cases where it is just a contest of survival between two opposed forces but that is the Horde vs. Alliance in the WoW universe.

The WoW villians are very not one-dimensional so you can't even say Blizzard is making it easy by just pointing to Old God taint.

The real thing that will make new encounters interesting are the encounters themselves and how it affects the game on a larger scale. Cataclysm looks to be one of the most revolutionary changes in the present story line ever in the WoW universe. I don't think it compares to the Sundering but that has always been ancient history.

However, oh crap, how droll... it is just another corrupted champion, Neltharion, the Earth-Warder (a.k.a. Deathwing) and this other one, Queen Azshara. *sigh* Blizzard why do you treat us so? We just got done beating Arthas, Illidan and Kaelthas... don't forget Vashj. Heck, just throw all the Eredar in there, too. /sarcasm

I personally am predicting the next corrupted champion will be Malfurion Stormrage.