The only problem with "new blood" is that, with the exception of singular rent-a-minions, new enemies take time to build up their character and level of malice. It's really difficult to do that in a single content patch, so new enemies on a grand scale are typically reserved for expansions.

@Iirdan:

I felt that Blizzard did a good job with 4.1 and the lead-in quests for ZG. We got to see what the Zandalari were up to and were given a reason to go in and kick their butts. The enemies were set-up, their methods known, and their motives... well their motives aren't really made clear, aside from the 4.1 launch trailer. There's definitely room for improvement, though.

That's true, but keep in mind that the Zandalari had been established beforehand. They were strong enough to give us the support to defeat Hakkar and the Gurubashi before; now that they were working with the remaining Gurubashi and the Amani, they were a clear threat, and one that had always been present.

If they were to introduce some new Ogre faction in an instance just south of Orgrimmar, that would be harder to sell convincingly.