What innovations are yet to come?
Certainly the sounds will be improved through the constant lowering of memory prices. More memory should mean more sounds and more importantly more variations of those sounds – more detailed pianos, more mega-type voices, etc. Technology already exists that will play subtle variations of sounds on playing the same note on the keyboard both, through playing at different velocities ala mega voices, but also randomly giving each key press a bit of spice.
The above is not specific to arrangers but will migrate from other (regular) keyboards and frankly anything that helps arrangers sound less cheesy is to be welcomed.
On the styles front, KARMA like ideas will affect the playing styles and as more progress is made on the softsynth front these will also be incorporated in arrangers. We may see the almost AI-like interfaces used to make style programming and editing easier.
Of course the other aspect is the unknown. Years ago we had the same debate over analogue synths and then out of the blue came FM. At the time I was bowled over; I hope to be again one day by the next generation of thinking.
On a final note, it’s possible economic or market forces may keep these innovations out of music shops but I tend to think people will always innovate (some of the best innovations happen by accident) and when it comes to technology the rules seems to be if it can be done it will be done and someone will sell it!