I think choosing distros from slightly more diverse backgrounds makes sense. Ubuntu includes most Debianisms including dpkg / apt-get, CentOS includes most RedHatisms including rpm / yum, while openSUSE is a third way (that I know little about).
Replacing openSUSE with Debian wouldn't be sufficiently differentiated from Ubuntu, IMO.
The inclusion of something inheriting from Linux From Scratch might be warranted for didactic purposes - just for understanding what bits a Unix is made out of, fundamentally.