New Navigation UI

This would definitively be useful. Whenever one of our apps moves to the new UI - especially one with a larger number of reports in it - we get a number of complaints from our users as they find it more difficult to navigate. It would be great to have the option to select which UI you want to use for each app, in order to provide the best user experience.

Would it even be an idea to allow any user to select the UI that suits them best?

This would be excellent, similar to the recent UI change for teams which allows users to rollback to classic mode after you realise the new one is full of bugs and no one likes it.