GC made an interesting point in one of the recent interviews I believe. He said that adding dungeons with a higher tier of gear mid expansion can actually create problems because unless they can add a ton of them (5+) players will very quickly burn out because that small number of dungeons to run.
While that is true (and was the main problem with ZA/ZG), what they could do is, along with any new dungeons, release upgraded versions of the expansion's original dungeons with upgraded loot. Since those dungeons were originally designed and itemized for the current expansion, tuning them up should be a relatively simple matter.
It's also possible to do a relatively small number of new dungeons 'right'. They really hit the ball out of the park with the ICC 5-mans, even though there were only 3 of them.
Part of that was just that they were great dungeons (I would kill for upgraded versions of them). It's obviously hard for the creative team to know beforehand whether their new dungeons will be well-received by the community (or else every raid would be as good as Ulduar and every dungeon would be as good as Pit of Saron). So I'm not going to say "just make better content."
Part of it, though, was that, unlike the Cata dungeons, they were in a shared random queue with all the Wrath dungeons, and they all gave the same reward to the emblem farmers; the only reason to try to spam the ICC dungeons specifically was for the gear drops themselves, and since you had to queue specifically to guarantee you'd get them, you were limited to once a day for the heroic versions. That's something they could
The Cata experiment of separate queues failed, so go back to the original (possibly with tuned-up 5.0 dungeons). Don't throw the baby out with the bathwater by eliminating new dungeons entirely.