- core services (Positron's core language system is an API, and R and Python are extensions) - native panes (you can contribute webview panes but they're slow!) - toolbars for other panels, or global toolbars - modal dialogs or any UI other than notifications and quick-pick lists - custom layouts
At a higher level, Positron is a platform that contains data science tools for _other_ extensions to use, and doesn't make sense as an extension itself. The R and Python extensions are the first two we built, but the platform is extensible to other languages.