Perhaps it's a new game based on the planet Pandora (Avatar)

This WILL be an expansion. Patches aren't trademarked. The application clearly states computer game (not website, trading cards, or anything like that).

With Pandaria we'll be getting the new continent that was promised, and obviously it's home to the Pandaren.

They won't be a playable race because it would simply be too much to expect 2 factions of Pandaren, one of them joining Horde, and the other one Alliance. Having it only accessible to either one would cause way too much unrest amongs players, so...

Most likely, though, we'll yet again have to make a decision like it was with Aldors and Scryers, to give us the usual reputation grind wrapped up in some story.

We'll also get another hero class more than likely. I'm pretty sure it's going to be the famous Monk as it would be the exact opposite of the Death Knights. Again a dedicated start area, with lots of Pandaren being used to guide us through it and instruct. They'll probably take a good amount of ideas from the whole Shaolin business, with funny references to more or less famous martial arts movies left and right.

Blizzard is back in business in China, so creating content that might not need to be completely redone to make it past censorship makes sense. It's going to be their biggest market soon.

I honestly don't expect too many surprises when the whole thing is officially announced at Blizzcon as usual.

... until we see people calling them "Pandarens"?

lol