I’d love it if SemVer was strongly recommended or even required.
Author URL should be optional.
Recommended additions, both mandatory:
# Compatibility: Last version of Discourse tested against.
# Contact: How can the developer(s) be reached?
I don’t know how exactly WordPress does its compatibility check that prompts the “this plugin has not been tested with your version”-notice when you install/update a plugin from within the admin panel, but I think it’s an incredibly useful heads-up and I figured this would be a decent first take on it.
(I think it’s based on the “Broken/Works” user vote data they gather after the plugin has been submitted. Still don’t know where they get the “Required: some version” and “Compatible up to: some version” though.)
As for contact, it’s just my belief that any self-respecting open source developer ought have an open line of communication of some sort. It wouldn’t have to be an e-mail, but there should be something put there that indicates how the developer would prefer to be contacted.