I was talking to my friend, and he was saying "there's a great launcher in gnome, you press the super key"... Which got me thinking, most users will think the start menu pops open a menu/launcher.
Maybe keep alt-esc but add a second keybind for it with the super key ?
I think it's a good idea to keep the SuperKey free. That way anyone can use it in the way it habitually does for them.
For instance on Deepin the SuperKey with one of the arrow keys switches virtual desktop instead of Alt+Ctrl and arrow.
Personally I'd opt for (considering the key often has a M$ Windows icon) letting it play diabolical laughter (Vincent Price comes to mind) and displaying the text: "Aren't you glad this isn't Windows anymore?".