There will still be raids and those will have "big baddies" of their own. The only difference is we won't have the lich king showing up in every other quest reminding us that for some reason we have to go joust and fight an undead beetle (again) before we can fight him.

Who knows? Maybe Garrosh and Varian end up being the big bads for now? Maybe we become the big bads? I mean in a world where war increases there are bound to be good and bad forces at work.
EX: The barrens for Ally. The head commander out there (totally forgot his name) is trying to finish it with only needed hostilities. Heck he even sends you to recapture the guys looting Camp TJ. But in the end, he gets made out to look like a bad guy, is killed and the actual bad guy takes over.
If the coming war is anything like Barrens then it is gonna get wild! (and be worth it to play on both sides to see each side of the story too! )

that Sha thing seems very interesting, it's like it feed on our war and violence. I think this x-pac is gonna be mindblowing. For starters, I'm really excited about just taking my mount and going exploring. It's gonna be a blast!! The new zones are damn gorgeous. I fell in love immediately. MoP can't arrive soon enough, I'd skip Deathwing completely to start playing right now!!

Well, I already voiced my theory on the Big Bad for the expansion before, but I'll say it again.

Anyone seen the movie 2001? What's the big bad there? HAL 9000.

But here is the question: what is the evil in HAL?

Answer: HAL is evil because of his programming. He does what he is commanded to do and nothing more. And people programmed the computer.

In short, the evil is us. We made the machine.

What is my point? The Panderan know the dangers of evil because it seems to physically manifest itself on the island, so if there is evil, it is only because we bring it with us. Therefore WE, Horde and Alliance, are the evil of the expansion.

It goes much further and deeper than that in my tinfoil hat theories, but you get the idea.