I think as long another platform isn't completely dominating, Dragon Quest will stay on Nintendo platforms. They're games with a broad appeal similiar to Mario and Pokemon so that's likely where the most fitting userbase is, and hardcore fans will get it regardless of platform.
Also I think and hope the next game will be handheld. They're much more comvienient to play that way, I doubt Hori cares much for making super high end HD games, and the series has never been never very cinematic. DQ8 looked nice but the realistic graphics hurt the gameplay. I'd prefer the next game more in the style of the PS2 DQV remake and there is no reason 3DS wouldn't be able to handle either style.