Add Unsplash Wallpaper as picture of the day provider. Unsplash provides high quality photos under public domain license.
Unsplash has very good wallpapers; in my opinion this automatically becomes the most attractive POTD plugin.
Keep in mind though that when I tried to add Unsplash wallpapers to KDE in D18078 I was told I need to contact the authors and have them re-licence their work for us...
Hopefully this issue can be avoided here as it's not exactly the same situation. You have a big +1 from me for the idea.
Photos on Unsplash are under CC0 (public domain) now. So here shouldn't be copyright issues.
I do think it is good to show photographers' credits. But that will require changes to POTD class and wallpaper widget.
Where did you read that Unsplash images were CC0? The license is not CC0 and is in fact non-free. Or are the pictures fetched at https://unsplash.com/wallpaper/1065396/desktop-wallpapers licensed explicitly under CC0?