Linux variety
Official ProtonVPN Linux App according to https://protonvpn.com/support/linux-vpn-setup/ supports the following distribution combinations:
* Debian GNOME
* Fedora GNOME
* Ubuntu GNOME
This is very one sided, each of them uses GNOME. I think adding variety with XFCE and KDE would be useful support.
Debian and Ubuntu are distributions in the same stream supporting more streams for example Arch would be useful.
Or at least target different package managers, the current supported list probably only uploads the package to APT and DNF.
Adding Pacman/AUR and maybe Zipper or Portage would be very useful to support the Linux communities.

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GizmoChicken commented
I'd be happy with an *official* flatpak and/or an *official* snap.
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Thomas commented
IMO Flatpak is the best solution to this since you can install it regardless of distro:
https://protonmail.uservoice.com/forums/945460-general-ideas/suggestions/46955335-official-flatpak-support-on-linux