I might be interested in this, if they switch up the gameplay a bit to keep things fresh. Though, I feel Final Fantasy is more prolific in the music department, and Dragon Quest games have a smaller number of tunes, some of which I'm not so sure would work well in a rhythm game. Hopefully, they'll at least use the music from the remakes instead of pushing in a bunch of NES tunes.
Either way though, I wouldn't buy this right away, and wait and see if they're going to release a special version later on. That is, if they even release it outside Japan, as this'll be the sixth Dragon Quest game for the 3DS and none of the others made it stateside.