This kind of thing could work for a few apps, say a color picker utility or a QR code generator etc.
Looking at the docs, it isn’t clear if apps can write to their own namespace (instead of writing to user folders directly), but if they can, we could expand the scope to games like supertuxkart, 2048 etc, which would then be able to save user milestones and progress in their own area - a bit like how Android apps do it
It’s a great start IMO, although admittedly there is still work to do. Flatpak atm bridges the gap with allowing new apps, requiring new libs, to run on older stable/LTS distros