I own a YC61 for 3 months and have some suggestions for improvements / new features for the next firmware update:

1-A better rotary speaker simulator (aka Leslie). We need a new simulator that is on the same level as what we have in Nord Electro. The current YC simulator is very bad, weird, bizarre when in fast, it doesn't look anything like the real thing. In fact it looks like rotary only has one horn instead of two, it looks like it's lame. Even the rotary speaker simulator of my Zoom MS70-CDR (Roto Closet) pedal sounds much more convincing and organic than we have on the YC.

2-More sounds using the FM engine, new leads, pads and some DX classics, such as Marimba and Flute.

3- At least one sample of real Human Choir, to mix with the piano sound.

4-More Synth samples like bass, leads, pure saw wave, square wave and pads.

5-The EG knob needs to work effectively on all sounds. The EG knob doesn't work on some sounds (or works very little), for example I tried to put a slow Attack / Release on Mellotron Violin sounds and noticed that there is no change.

6-The TAP TEMPO button can be used for all delay effects, not just in tempo delay efx.

These are my most important suggestions, you can comment and add yours. Let's cross our fingers for Yamaha to read these improvement needs and put them into practice in the next OS firmware update.
Julio 😉