Hi Scott,
The likelihood of a manufacturer coming up with an instrument that has the features you described is very slim IMO. Although you do have good ideas; I had suggested more of a modular approach with a past employer quite a while ago but got shot down. There just isn't a growing market for arranger keyboards.
All hope is not lost however. Personally I think the way to go will be a computer based system. I have been (and still am) against using a computer on stage as my only sound source, but with the newer tablet PC's, the iPad, etc. it is getting much easier to accept the idea. So I think the route manufacturers need to explore is software based arranger apps integrated into a complete DAW's. Actually this can be done now using multiple programs. But I would like to see a single application to run on a touch screen type PC that included everything; arranger features, modeled acoustic and analog sounds, looped live tracks, sampling, full sequencing/editing, etc. Then by using one of the newer MIDI controller keyboards with assignable faders and buttons, you could decide what features to access via the touch screen and which ones you would rather have on actual sliders/buttons. Some of the newer controllers have audio inputs as well so you could plug a microphone into the controller and have access to a vocal harmonizer and EFX processing that would be included in the application. Plus you would have a choice of a 61, 76 or 88 note keybed or for those who really want to travel light, there are even 49 or less keys.
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Wm. David McMahan
The Modulators [This message has been edited by WDMcM (edited 12-13-2010).]