Ideal arranger features:

-- No keyborard at all. But it is not a rack: it full size and mounted on top of you preferred master keyboard. It can be up to 3 keyboards attached (up to 3-manual instrument).

-- Style morphing

-- multi-pads

-- soloists

-- Karma function for all parts

-- performance rendering for all parts (swing, nuances etc.)

-- vocal processor (harmony)

-- contemporary sound engine: samples, virtual sinth and VariPhrase-like processing. Importing sound streams from computers (USB).

-- music database with tons and tons of presets (styles and registration). Online exchange point of presets with some kind of collaborative filtering.

Due to presets it cannot be done completely from existing modules -- it should be integrity in sounds.

Have "style and pad compatibility feature" with Tyros/PSR, VA, KN, SD, Genesys etc. but I do not know how to integrate it to Music database (General Midi Styles? Oh, no!).

-- new modules and updates loaded via internet (like Creamware Noah) always with additions to MusicDatabase.

-- huge hard drive -- and sequencer is always on.

-- 7.1 sound (not merely stereo)

-- CD/DVD-audio-burner and MP-3/OGG encoding

-- computer integration (a lot of it own software and configuration files to existing software)