Maybe walk before you try running?
A full software arranger is a mighty deep project, and Dan has already beaten you to the punch, making your efforts a less than guaranteed commercial success.
However, there's something that many arranger players could use that is FAR easier to program, and quite useful to a broad variety of arranger players... and nobody at ALL currently makes.
How about an add-on Chord Sequencer?
This is basically a program that would sync to the arranger's MIDI clock, scan the NTA channel (the Note to Arranger channel, which outputs whatever Part and Channel is being sent to the chord recognition engine - it's common on most decent arrangers) and on command, loop the input seamlessly with the output. Thus, you hit record WHILE you are playing, the program grabs the chords ONLY of what you play, then when you trigger 'Play', sends them back in again, to take over your LH chores, seamlessly.
Currently, only the PA3x and the BK-9 have one of these, they are incredibly useful for players that have other uses for their LH more than simple repetition of the same chords over and over, and no-one makes a software version so that those of us with arrangers without the feature could easily add it to arrangers we already have.
I would pay good money for this (I'd even buy a Windows notebook to run it on!
) and I am fairly sure many Yamaha and older arranger users would be very interested.
A full featured software arranger may be a HUGE bite to swallow, but this would give you much experience, target a market segment not already addressed, and give you insight into whether the full project is possible, for you...