Would it be trivial to design a drive that can be switched into a double-speed half-capacity mode?
There's a word for it... "Velociraptor".
There's even a word for a drive that's "triple" speed... "Cheetah".
In any case, you wouldn't decrease the capacity on account of the faster rotational speed... you'd just use a faster DSP capable of doing its thing in less than half the time as a slower drive. From what I recall, the Cheetah's storage density per platter was basically the same as any other 2.5" drive.
SSDs obviously made the highest-performance spinning disks almost irrelevant, but personally, I used to think it would have been awesome if Seagate had taken the Cheetah platform, added two more independent sets of actuators and read/write heads, and wired it all up to look like 3 SCSI drives with sequential SCSI IDs so you could have single-drive RAID-5 performance in a luggable laptop (think: inch-thick Alienware/Sager/Clevo) or SFF desktop. Heat would be an issue... but really, a Cheetah didn't throw off any more heat than the mini-PCIe discrete video cards found in some gamer/mobile-workstation laptops now. In MY laptop, at least, the GPU's cooling system is bigger than the CPU's.
One thing I'd LOVE to see, and even think there's a market for, would be a single-platter drive suitable for mounting in the optical bay of mobile workstation laptops (say, 120mm diameter, 7mm or thinner). I rarely use optical discs, but having another 4tb or so that's always with me would be nice to have. Basically, it would be 7mm thick Quantum Bigfoot from the late 90s... and Jesus, with that much diameter per platter, just imagine how many terabytes you could pack into a multi-platter drive that fully-consumed a 5.25" quarter-height drive bay. It's almost scary to think about something like a 256-tb 5.25" single-bay hard drive.
I'm also kind of surprised that nobody ever made a thin-but-3.5" drive for laptops (which would obviously need a larger drive bay... but modern laptops, even thin ones, have SHOCKING amounts of horizontal acreage under the keyboard that could easily be put to good use for bigger cheap drives).