User defined/created maintenance page.

Hey

In the Maintenance screen.
Below the Enter a Message field. Could be an option to select a page or add a new page that can become the maintenance screen that will be shown on the frontend when the Maintenance is activated. This means the user can create their own maintenance page and select to have that seen on the frontend instead of the default seen today.

Could be something like this:
Select a page or create a new page to become your maintenance page. The selected page will be seen when the maintenance setting is active.