Marshmallow can format the SD Card with ext4 (I think) and encrypt it (as all storage will be with Marshmallow) so it can only be used inside that phone. Then, the SD Card can be used/accessed like the internal memory.
But this will not be possible (as far as I know) with USB storage. I just tested this with my Nexus 5. Android just offers me to format a USB Stick for “external storage” which will not be writable without the Access Framework.