Just put more drives into the box. You don't want bigger disks because of how fast they spin.
The only 'obvious' improvement you can make is having more arms or figuring out some way of micro-aligning multiple heads at the same time on a single arm. There's not much benefit from larger single units.
You can already fit way too much SSD into a 3.5 inch drive, and nobody will ever buy it because they want more ports and performance per dollar. There's no benefit for either technology to go up to 5.25 inches.
The only 'obvious' improvement you can make is having more arms or figuring out some way of micro-aligning multiple heads at the same time on a single arm. There's not much benefit from larger single units.