You could do a lot with hackable HD firmware. For example recover data from bricked disks or find bugs or make sure that disk you're about to RMA is infact fully wiped, etc etc. That would be especially useful now that SSD's have "turn into brick" failure modes and frequent firmware bugs that trigger them. Other ideas: adding crypto or RAID-friendly knobs, making the filesystem and the on-SSD flash fs aware of each other and parametrize accordingly, etc etc. In fact sounds like Apple's latest Fusion Drive is a thing that requires disk firmware and filesystem to cooperate.

The Alphamosaic processor (branded VideoCore in its Broadcom incarnation) has also been marketed as a network and security accelerator in its previous life... It might well be useful for all kinds of stuff.