An AU host version of KRFT so you wouldn't be limited by the instruments available in KRFT yet you'd be able to leverage its controls and GUI is my wish.
KRFT could send the following to the AU apps: Notes, CC messages, preset changes. Being able to sequence all of these is desirable.
You'd open up the AU app, set controls up and then close it so it's just played via the KRFT surface controls. XY pads where you select which parameters and/or their inverse for each axis would be essential to maximize the potential. A third axis for pressure sensitivity on devices (recent phones) which support this would round things off.
AU effect apps could also be used to alter the built in sounds of KRFT too.
On the phone, the AU host app version of KRFT could really give people an opportunity to create more involved setups that can be easily played.
I think IAA apps are too inconsistent in how they're implemented to create a reliable user experience when hosted in KRFT versus hosting AU apps.