I've posted about this before https://forum.kde.org/viewtopic.php?f=17&t=174451&p=453789&hilit=jgmenu#p453789. Since I'm a hardcore openbox user, I'd wish someone who knows how to make plasma menus came up with something similar.
This is jgmenu and in terms of workspeed, these kind of floating menus have many advantages over the..um..regular plasma menus. Jgmenu is highly configurable and it supports, for example, openbox pipemenus. It can do tons of layouts etc... Something similar, in a true qt/plasma version could be great. Something to put on the panel ( in screenshot that person icon next to those headphones) or, if you customize the plasma desktop right click, in a true floating manner.https://imgur.com/a/Xyb2NS2

I just like to point out that in order to get the menu to match the gtk theme, you need any theme, other than breeze. Because since that halfbaked highlight color implementation, a lot of stuff ( not only gtk, but also icons and what not). If they really insist on adding **** "new" stuff to releases, at least provide some kind of off-switch. Anywhooo...You can't do a QT_STYLE_OVERRIDE= to, say, lightly. So, either a theme from the web or a self-made (oomox) one that matches the color-scheme you're using. Total mess, yet do-able.
Second screenie: Lightly application style for regular windows. Kvantum override for audacious player. Qt style override for fsearch and a custom gtk for certain apps, like the menu. Mess.https://imgur.com/a/Am233KN