I think the main thing that all arranger manufacturers can do to make them easier to operate is to ensure that the registration data memorizes the COMPLETE state of the instrument. No more 'this data is stored here', 'that data is stored there', and 'the MIDI setup is stored here'...
If you want to go from one registration using internal sounds to a reg. that uses a few external modules (where most arrangers will make you change MIDI configurations), or change chord recognition modes mid song, or just about anything that currently involves at least two separate load functions, it should ALL be one button.
ONE registration to bind them all...
And another thing...? Make sure that changing from ANY registration to any other registration is completely seamless. No voices get cut off, no glitches in the sound while parameters change, no hiccups in timing, no nothing. You should be able to change EVERYTHING on the fly without the slightest loss of flow.
Lastly... FAR more comprehensive 'HOLD' functions. You should be able to put a HOLD on just about anything, from Tones to Split points, to recognition modes, to tempi, and not have OTS over-ride or new registration change your 'held' parameters. Most arrangers can hold SOME parameters, but none (AFAIK) give you a comprehensive set. Everyone uses an arranger differently, expecting one limited set to cover ALL the possibilities is unrealistic.
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An arranger is just a tool. What matters is what you build with it..!