For example, I'm using Debian, and I think we could learn a thing or two from Mint about how to make it "friendlier" for new users. I often see Mint recommended to new users, but rarely Debian, which has a goal to be "the universal operating system".
I also think we could learn website design from.. looks at notes ..everyone else.
Fedora, NixOS and Void need a proper wiki like Arch
Most distros could also learn from Arch and create something similar to the AUR. Nix is going in the right direction.
And I guess almost all distros could learn from Artix and Devuan and reconsider if systemd is the right choice.