A feature like this already exists.
If you click on a playlist, you will find an "Options" button int the right top corner. Clicking on it, shows different options and one of them is "Play when idle". This will let the bot play this playlist, each time no other song is played.
You can do the same with tracks. Just click on the options button of a track and click on "Play when idle".
But this isn't working so for radio stations. So the options could be implemented for radio stations as well.