In response to a forum thread started by Cedrickt on the beta forums, Valnorth stated that the Dragonblight quest line ends at the Wrathgate cinematic on the beta. As yet, there's no further information. We don't know if there's any intention to add in entirely new quests, but I expect not; trying to gibe Garrosh as Warchief with the events of Wrathgate (to which much of his policy toward Sylvanas is a direct response) would be very difficult indeed.
Go play it while you can. It's a lot of fun.
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