More control for panel transparency

Will it be possible to add more control to it and change transparency gradually?

I don't believe that's something we'll add for Budgie 10.x but I'm hopeful it's a feature we'll have for Budgie 11.

I recall you can set panel to have a solid color and adjust the alpha value via CSS.

Add this to your ~/.config/gtk-3.0/gtk.css for a black panel with 50% alpha:

.budgie-panel {
    background-image: none;
    background-color: rgba(0,0,0,0.5);
}