I have a setup with KDE Plasma and i3 that I've been absolutely loving, except for the fact that the windows key no longer works to open Plasma's application launcher like it does with Kwin. I did some research and found out that normally Plasma handles it by assigning Alt+F1 to the launcher and then using ksuperkey to allow a single windows key press to emulate that action. However, ksuperkey depends on Kwin to work - and that's been replaced by i3. I haven't found another way to do this.
Also, I mostly followed this when setting everything up.