I'm talking of allowing apps to mark things (like the WhatsApp data previously mentioned) to only be accessible by itself or some other app(s). As for FAT, I s

I'm talking of allowing apps to mark things (like the WhatsApp data previously mentioned) to only be accessible by itself or some other app(s). As for FAT, I said in another comment that Google has already imposed MTP on us and Android's adoptable storage uses ext4 anyway, so they could easily require it on all removable storage where application data is stored.