Well...you said no reference to the past, but the only thing I can think of right now is something new, but also an improvement over a feature that already exists: to me the ultimate arranger must have the ability to play a STEREO sampled loop in sync with the style.
Today we have available many excellent CDs with tons of great sampled loops, especially of drums and percussions; to my knowledge there are three brands of arrangers that can play a loop in sync with a style: Ketron (with more than one model), Korg (with the Pa1X pro) and Roland (with the G-70, that unfortunately can play only loops found on the expansion boards).
However, all these arrangers can play only MONO loops and this is a shame, because the sound quality of a mono loop is much lower, compared with a stereo loop.
The first arranger that will play a stereo loop in sync with a style will be my next arranger, no matter how pricey it will be.

[This message has been edited by Dreamer (edited 02-18-2006).]