I somewhat recently ran across VanillaOS, which I have only really had time to install and play around with for a few minutes, but it seems really cool. A very brief overview is that it is a sort-of-but-not-really immutable OS that leans very heavily on containerization to allow you to install packages from any other distro in a seamless-to-the-user way. So you can install an application (cli or GUI) from an ubuntu repo and use it along side an application from an arch repo. It's ubuntu-based, but according to the info on that link, the next release switches to being debian-based.
I mostly use ChromeOS these days-- well, I guess technically I mostly use SteamOS these days-- so I don't have a lot of hands-on experience with VanillaOS, but I found the concept really cool and from a few minutes of playing around with it, it seemed to work pretty well with respect to the containerization stuff.