That's something I'm still figuring out. In particular, I want plugins that add extra content to pages to be able to collaborate with each other. I have an open issue for that here: https://github.com/simonw/datasette/issues/1191

I'm not too worried about extra template vars overrides though - that will only be a problem if two plugins accidentally use the same variable name. Maybe I should encourage plugins to use a namespace based on their name in the documentation though.