Will it be possible to add more control to it and change transparency gradually?
More control for panel transparency
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.
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- Edited
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);
}