My Top 3
1. Styles
Alphabetical order option
Move ALL Country styles under one category, lots of room in "World"
2. Utility/Parameter Lock
Add more options (including locking in user Live Control settings)
3. Bluetooth
Two way connection. (Imagine being able to play your keyboard to a Sonos, Bose, etc. Bluetooth speaker system)
All that being said, many of the improvements mentioned, including my own, will probably not happen in an upgrade. Many are hardware related, and not possible for Yamaha to implement. Think of the SX-900 as a computer, you can only do so much in improvements , without physically altering the mechanics.
However, like a computer, many software related improvements are possible. We'll see what upgrades Yamaha chooses, hopefully your request is included.
In the meantime, we are all stuck at home, so for me, I'm enjoying my SX-900, as is...a fun keyboard