I think that you should deal with chord recognition computer way: implement Roland and Yamaha and Korg and Ketron algorithm, then permit to musician select most appropriate to her.

Main difficulties with software arranger is variety of sounds and effects. This mean that impossible to create "factory presets" and this lead to situation similar that with G70 that is very good instrument exept buggy factory presets...

My wish list to software arranger -- style morphing, soloists/riffs(pads), style part variator (karma-like or continuator-like), possibility play native (not converted) styles from several vendors (Yamaha, Roland etc.). All other things we can get from synthesizers, not arrangers.