I can't remember what I did .. I need a new, minimalistic switcher (Alt+Tab)
The new Alt + Tab functionality is from Gala. If you can't remember what you did you could always reinstall Gala and that might help. From the terminal:
sudo apt reinstall gala
Whats going on here:
sudo only someone with admin access can directly install/remove/reinstall software. This will ask for your admin password.
apt apt is the underlying software that Ubuntu and by extension elementary use for managing system software packages.
reinstall This command tells apt that we want to reinstall something thats already on our computer
gala The name of the package that we want to reinstall.
So to break it down we are telling the computer: Ask for admin access for the package manager apt, and then in apt we want to reinstall the package gala.