Everytime I do this quest I feel almost shoehorned if you would, into doing the bidding of Valdar. Is it just me, or does it seem kind of a leap of faith to trust in some random ghost who claims that an ancient evil is about to be freed? Sure, Hevnoraak is a dragon priest, but who's to say that Hevnoraak can't help you out or reward you moderately for removing Valdar.
Considering that Hevnoraak seems to be stronger than the guardian keeping him imprisoned, it just seems like a ripoff to just get a quest reward involving only some gold, loot, a cool staff, and a mask. A dragon priest probably would know all kinds of ancient forgotten knowledge that they might be willing to part with for such a generous donation of removing Valdar.
I know I'm preaching to the choir here, what with all the ideas on more quests with multiple endings and all, but I wonder how different it would have been if we could temporarily side with Hevnoraak? It wouldn't be too much of a stretch since we can already side with cannibals, forsworn, daedra, and the like.
How different would it have been? I really do wonder.