If you like the HD’s to compete with current SSD’s, at least for the short tem, you may want to have a double, triple or quad actuator arm in a small 2.5 factor disk. With a smarter controller, you could have the benefit of deciding to use them all for reading, writing, or a combination of reading and writing at the same time.
Rotational Latency will be divided between the arms
Translational latency will be minimized if the design allows for smart predictive landing zone of each arm - whichever is closer to the data can access it.
Putting various arms like this will probably behave better than current RAID arrangements, since the internal architecture will not be limited to a static RAID layout (it’s the same physical disk!), but will be smarter since it will know when all heads will be reading (possibly from same cylinder if necessary), writing or doing both and deciding when the arms should execute the operation.
This does not change the current technology at the low level: it merely parallelizes a proven solution, and makes it smarter.
If you add a bigger buffer to the disk, it will be hard to beat.