I have been able to use TJ successfully with external apps like StepPolyArp etc!
One thing that would help a lot is to have a call out column of MIDI Controller Buttons, (much like the scale button presets) letting users define MULTIPLE MIDI events like Notes, CC's, Program Changes...etc each with it's destination App/MIDI channel(much like most of the MIDI Controller Surface apps like MIDI Studio...etc).
This would solve quite a few of iOS musicians' problems:
-chord playing(each button can output multiple notes)
-changing programs/presets in other apps
-changing various CC's in other apps
-starting/stopping/continue playing in other apps
Maybe adding slightly longer slider bars(for volume...etc) as well as square shapes.
One problem with playing in iOS live is that it is very difficult to control external apps from within one app as it is difficult to switch apps while playing(which often cause stuttering anyway), if such buttons are incorporated, using TJ as the main MIDI controller app would be absolutely fantastic(which it already is really....
)