Since Diki didn't suggest it, I will:
A CHORD SEQUENCER!!!

Basically I am looking for a "chord bank" where you can pre-load the chord/bass entries that feed the arranger. During playback you can still change styles, variations, fills etc. on the fly. Playing over a chord bank or chord sequencer represents a halfway point between playing the chords in real time, and playing over a stored arrangement. It leaves your left hand (mostly) free for fills, variations and working the pitch bender.

This feature is so incredibly useful, I can't believe it was allowed to disappear back in 2001 with the lowly PSR-225. C'mon Yammy bring it back on the "PSR-S910". -Ted

[This message has been edited by TedS (edited 07-08-2008).]