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Welcome,

Basically I propose to add more advanced configuration options to control panel visibility with GUI similar to kwin rules. For example:

Panel is visible only on a certain activity.
Panel is visible only when a certain application is running.
Panel is visible only if the current window is maximized.

For instance this way we would be able to make a GIMP panel that will show up only when GIMP is running and will contain links to tutorials, recently edited files and things like this. This way We can somehow boost effectiveness while working with the help of plasma shell applets.

What about "icons making" activity with a panel only visible there with additional tools like color picker?

What about a top panel containing current application control and menubar that will be visible only if window is maximized?

I know that activities allow us to have different widgets on the desktop, but using plasma dashboard is not always handy.

I think we do need per-activity panels, or some other mechanism for displaying per-activity widgets that doesn't get blocked by windows and is not the dashboard.
Arguably your GIMP suggestion may be accomplished by having a dedicated GIMP activity (accompanied by the per-activity panels).
I do like the third option because there it will assist in maximizing screen real-estate with maximized windows - displaying elements that were previously in the top of the window in a more (vertically) concise format.