Hi Philipp,
A freind of mine ported the CSound API to objective C for iOS, and can run csound using the normal API there. Somebody else used libPD in a granular-synthesis app, using objective-C / C / PD. OpenFrameworks is pretty cool for this type of thing, but if you have to use objective-C there's some bindings for other things around.
If you're in need of ideas, let us know approx what area of stuff you're interested in and then we can hit you with some brainstorming?
Also, if you design the app carefully, it could be trivial to port to linux/android/embedded or something.
HTH, -Harry