By default, KWin does not have shortcuts for the Increase/Decrease Opacity of windows but it does come in handy often for some people, such as myself so I suggest they be set to the following. Increase Opacity=Meta+Alt+= Decrease Opacity=Meta+Alt+- This is similar to how the Zoom function works with Plasma but adds the extra Alt modifier. This is the setting I use for my workflow and I use it often when podcasting so others might find it useful and it doesn't negatively affect anything as those shortcuts I'm suggesting are not utilized for anything at the moment. I was told to create a bug report for this instead of a Phabricator task so this is that bug report. (reference: https://phabricator.kde.org/T10568)
We have it possible for users to add shortcuts for this action. See "Increase opacity of active window by 5%" This is a very niche feature, and adding global shortcuts takes potential global shortcuts away from other places. I don't see a need to set a default.
The need is discoverability and better user experience. Saving it for what "could be" seems very wasteful to me. The vast majority of people have no idea that his feature even exists because the discoverability is 100% exclusive to digging into the shortcuts in System Settings. The window context menu (Alt+F3) does not offer this as an option to use unlike other KWin actions so without it being preset and people telling others to check it out the only way to know it exists or even see a demo of it as a user is to manually go into System Settings -> Shortcuts -> Global Shortcuts -> KWin -> dig through all of these to eventually find the Opacity actions -> Manually set both them. Yes, it is somewhat self-evident for what they are intended to do *but* not everyone knows what opacity means and the manual involvement required is just unnecessary barrier to entry.