Pixabay and Openverse are both massive repos of graphical assets that are free to use. They both have an API.
A really nice addition to Pro would be an integration with one or both of these services. Probably as an included plugin because you’d want a ‘browse Pixabay / Openverse’ button in the media library. That way you could browse and use images from anywhere there’s an image selector in Pro.
Have you tried out the Wordpress plugins they have? I would start there because it sounds like they are doing exactly what you would like it to do. Let me know what you think or if they aren’t working in Pro.
https://pixabay.com/blog/posts/wordpress-plugin-pixabay-images-3.0-136/
Free Assets Library – Openverse Another plugin
I’ve checked the one from Pixabay, but it looks like abandonware. The second one looks promising and I somehow missed it while looking around a bit. I’ll have a look 
@JvP Not sure at all about the quality of the plugin but there is this guy here as wel – https://wordpress.org/plugins/instant-images/ – might be looking into this myself now for future builds. Thanks for the tip!
Looks like it covers Unsplash, Pixabay, and Pexels + Good reviews + Updated within the last month. 
Thanks @DoncoMarketing that one looks like a decent option as well. Will have to compare to see which one is the best fit. Thanks!