But presumably an extension for wallpapers shouldn't be accepted: If you can get rid of an entire class of extension code by supporting wallpapers... let people

But presumably an extension for wallpapers shouldn't be accepted: If you can get rid of an entire class of extension code by supporting wallpapers... let people set their new tab wallpaper, block all new tab extensions that say "wallpaper", and tada, entire vector for malware gone. Extensions are a massive security vulnerability: They often have access to your web browsing activity, and they sit after TLS termination, inside your browser's trusted environment. Browser extensions should be rejected by default, and have to constantly justify their existence. Pointless extensions (including "cloud to butt", sorry guys), should never have been accepted.