You're spot on about the MZ-2000 AJ in terms of synth power. That board is quite dated by today's standards, yet it still blows the Tyros 2 and PSR-3000 clean out of the water in terms of (internal) voice editing. If I remember correctly the MZ lets you have up to 4 osc, along with numerous editing options per osc. The MZ's effects weren't too bad either.

I'm still keeping my fingers crossed for a Casio MZ-3000--I won't hold my breath though

I do however feel the strongest "synth arranger" (closed system that is) has got to be the Korg PA series.