Hi,
there already is a condensed iOS version of Halion Sonic, it’s called Micro Sonic. That is provided as a part of Cubasis: Cubasis User Guide: Micro Sonic
However, I don’t know whether there are plans regarding opening the iOS port to non-factory content.
Only someone from Steinberg could comment on that. But it would definitely be a great move, to enable users to create content for Cubasis (iOS) as well.
Cheers,
Mike