I'd love to have mass storage in my future Jolla, to be able to mount btrfs, ext4 or f2fs. I'd love too to have drivers for Windows/OS X for btrfs and f2fs.

About FAT and ExFAT, f*ck them. I suppose that it isn't needed to talk here about why they s*ck. I have some external disks with a minimal FAT patition just to carry an ext2 driver for Windows, and a big ext3 partition. I hope F2FS (or any other free (libre) filesystem) win the battle to ExFAT.