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Hero
Yup. I hope they do Ferrous Dragons next. Or Gem.
I can't speak with any authority on what WotC may or may not do, but I wouldn't count on this; to the best of my knowledge, such dragon types don't exist--or at least haven't been confirmed as existing--in 4E at all.
According to the MM, the five dragon families acknowledged in 4E (so far, anyway) are the chromatic, metallic, planar, scourge (aka linnorms), and catastrophic dragons.
While the planar dragons aren't really a family unto themselves--some of them appeared as variants or relatives of the chromatics in Draconomicon I--I'd expect that we'd see books on the other two types already announced before there'd be any chance of covering types not yet introduced.
(Acknowledgment of personal bias: I personally never cared for the gem dragons, and I think the minimal descriptions we've gotten of the scourge and catastrophic dragons sound really cool, so the above is not only what I expect, but also what I hope. That fact aside, however, I also believe that my reasoning is fairly solid on this. I can't imagine they'd introduce a dragon type into the game and then ignore it in favor of a new type that hasn't yet gotten so much as a name check in the current edition.)