It rather sounds as if you are going for an offline 'BIAB' approach?
I don't plan on doing things the way Yamaha does. More the way band in a box does.
If so, this isn't really an 'arranger', at least in the way we all use the word. And BIAB has been around a VERY long time and is incredibly mature and complex. You sure you want to go up against that?
As I said earlier... If you have the programming skills for this kind of work, why not find something (doesn't HAVE to be my suggestion!) that no-one is already doing?