"+" and "-" Keys to zoom in and out seem quiet natural and would allow for more control without the mouse.
Migrated to kwin Resetting version
What specifically is the request here? We already have Meta+"+" and Meta+"-" as default shortcuts, and assigning + and - as default shortcuts would not be ok.
The issue was about controlling the cube's distance with hotkeys additionally to the scroll wheel, but I don't consider it important on second s sight.
In any case, I would have thought of to use "CTRL + +/-"