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.
Most distros could also learn from Arch and create something similar to the AUR.
i've seen Void's xbps-src tool compared to the AUR multiple times in /r/voidlinux (and i guess it's like a decentralized AUR?? you can build+install pkgs from source using the package manager, sure, but there's no one big diy xbps packages registry like aur.archlinux.org for Void) and while i don't really see it, if you follow that train of thought, void's pretty set in the "right direction" :D