In this kind of hierarchy, I'd personally say fedora is third gen (due to it being so similar to Redhat) and that nixOS is fourth gen. Both came out around the same time, but took such vastly different routes with different kinds of itch scratching.
Gentoo and Arch are different takes on the same ways to build a distro.
Gentoo took the FreeBSD ports tree model and applied it to Linux: still relatively conventional packages, but they're source and you compile the whole thing each time. Arch, still conventional packages, but no fixed release cycle.
Nix throws all that, and the directory tree, out.
Slackware: tarballs are good enough, they're all my dad and grandad ever needed.
RHL: we'll have a package format, where packages can depend on others.
Debian: we'll take the RH idea, but make a tool that can go fetch and install what's needed by what you asked for.
Nix: you don't need to worry about stuff like packages or where stuff is. Tell us what you want and we will make it happen. (But you won't like the disk layout, so don't look.)
Guix: we'll do Nix but with Scheme.
AppImage: hey, you know Acorn did that apps-as-bundles thing first? And it's on Linux as ROX? What if we just zip up the bundles and mount them on demand?
Flatpak: that sounds too hard, dude. But we all agree Git's cool, right? So, what if we could, like, distribute apps over Git?
Gobo: all this packaging and dependency stuff is BS because you're still using a disk layout you improvised on the fly on some 1960s minicomputer with like 20 tiny little hard disks. Here, let's do a clean modern layout like NeXTstep did, but for the whole OS, then you don't really need a packaging tool any more. It's all just bundles, all the way down. But they're versioned. Just copy what you need.
(Entire rest of Linux world) Waah! But mah FHS! If I don't have my FHS then I won't compile!
Gobo: OK, OK, I'll fake it with symlinks for you, then hide it.
Snap: hey, app bundles sounds good, but let's compress them into single files and mount them when needed. Bunch of symlinks and you'll hardly be able to see the joins.
