As a music producer myself I would NEVER use a tablet for making music. It would be a nightmare to do postproduction and anyway you can't connect any pro soundcard. But an app that could export some midi partitions that I would write would be pretty cool. So I can write melodies on the go and then produce when I'm back in the studio. Even better, an app made by a music software company (Steinberg, Ableton, etc) that would directly sync with your projects on your desktop with a cloud service or something would be amazing but also very complicated to operate as your desktop may have lots of customisation such as VSTs and others.