Nomacs goes in this direction, but is not exactly what you asked.
I had the very same question for a long time now since the best organization of pictures is a tree directory, not a set of tags in database you risk to loose whenever you reinstall your linux.
Nomacs has an option in File menu called "Scan folder recursive" that enables jumping from a subfolder to another, in view mode, in slideshow mode or whatever. It doesn't show all the thumbnails in a same place though.
In panels menu, "Thumbnail preview" allows you to view very quickly your folder contents. It just needs a browsing option that shows folders next to another in a timeline fashion. Let's make a feature request, the devs are quite responsive :)