To this day, there are still numerous options that don't have a GUI. You either need to use Terminal commands, or something like TinkerTool to get at them. Things like a third minimization animation (suck-in), having transparent Dock icons when you hide the app, and so on. These should be exposed to the user, all of them are supported by the OS and there's no real reason they should remain hidden.
Beyond that, I would expect less new features and more iteration. I feel things like Stage Manager could be improved a bit. There are other apps and things here and there that don't allow the user to customize them.