I would like to second Starkeeper's suggestion of making a software arranger that is also a VST host. This would greatly simply things from a user's standpoint. A VST host would have soundfont capability because it could host rgcaudio's free sfz soundfount player.

Here is an idea for chord recognition. Allow the user to set a scale for one-finger chord recognition. If a user selects no scale, all one finger chords are major. If a scale is selected, it is assumed that a one finger chord calls for a third from the scale. For example, if a Cmaj scale is selected, c,f,g, trigger major chords, while d,e,a,b trigger minor chords. If the player wants a different chord, it can be more fully fingered--for example, in Cmaj scale, c-eb triggers the Cm chord.

Seventh chords can be triggered by playing the real note to the left. In other words, 2nd intervals are interpreted as sevenths based on the higher note of the interval. Thus, in Cmaj scale, Bb-C is a Cmaj7th.