According to series director Yuji Horii a brand new Dragon Quest game is in development at Square Enix. He had mentioned this at the Niconico Super Conference 3. No other details were given besides the fact that a new game is in the works.
It seems that the game is in its very early stages of development and the other members on the Dragon Quest team seemed surprised by this news so don't expect a big announcement in the near future.
Will the next Dragon Quest game be another MMO like X or will it be a next generation RPG that matches the likes of Dragon Quest VIII? Only time will tell.