I posted some info about DQ heroes localization earlier today. This is fabulous news.
As far as Theatrhythm and any other recent DQ game (X, VII 3D, etc) nothing has been announced. Im pretty sure that X is a no go, VII 3D really should happen and rhythm is probably a no as well.
Dragon Quest IX sold a little over 1 million copies in the west.
Dragon Quest IV DS games took in 300,000 or less
Dragon Quest Monsters: Joker - which they heavily advertised did even worse.
Even lower sales for the other Dragon Quest games like V and VI
I think Square Enix is done for now sending us DQ games for Nintendo consoles.
I thought there might be a chance to see Theatrhythm because of how well Final Fantasy sold, but I've heard there are even less tracks in the new game. So I doubt we will see it unless they wait until a "Curtain Call" style re-release happens.
I'm almost certain we won't see Dragon Quest X (especially since it is a Nintendo/PC exclusive). I realize it was also released on Android or something, but that won't happen here in the west either.