Cem Olcay has released an interval based AUv3 MIDI keyboard app, In Theory for iOS. This is what he has to say about it...
With the keypad buttons you can play and walk through the notes in a scale. +1 button plays the next note in the scale, +2 button plays two notes after in the scale and so on. Also you can create rich chord voices with the additional chord intervals.
It supports MIDI keyboard input for playing the intervals, MIDI CC input for toggling the chord intervals and AU params for everything else. You can also sequence it with MIDI sequencers.
Pricing and Availability:
$4.99 at the App Store
Like a having your own synth museum
Big savings till Dec 1st
Mix transitions made simple, yet so nifty
The Gas is strong with this one...