Already posted a suggestion on bugs.kde.org but I don’t know if that’s the right place for it so I’ll make my case here.
By default, the panel’s opacity is set to adaptive, which makes sense: this is beautiful against the desktop background, but may look out of place when a window is maximized, due to the translucent background peeking against a presumably opaque window.
However, whether a window is maximized or not, the widgets could still be translucent by default, as they do not suffer this problem: a translucent widget against a fullscreen window would simply look like it “fits in” - for example, using a translucent menu while scrolling the youtube homepage makes for a great visual effect to my eyes.
Currently, widgets get opaque with the panel when a window is maximized, effectively making the widget transparency effect invisible most of the time, which I find to be a missed opportunity.
I would thus like to suggest keeping the widgets always translucent in adaptive mode so they blend in with whatever window is behind, making better use of KDE’s beautiful blur effect.
Following the same logic, the panel could also be made translucent when auto-hidden, as there would be nothing to “clash” against since whatever window may be maximized behind it would also be covering the background anyways.
Of course, people seeking this effect currently can simply go and make the panel translucent manually, but making this the default would allow new users to benefit from this as well, assuming such an effect is deemed desirable by the team.