This question (Why don't digital cameras have better dynamic ranges?) combined with a typical sunset photo got me thinking...
Exposure differences between sky and ground in a sunset shot are usually, by me at least, solved with a grad. ND.
But, what I'm wondering is...is it technologically possible to have the sensor apply, say ISO 100 to one half of the sensor while applying, say ISO 400 to the other half?
An ND grad is cheaper and bracketing is cheaper still, almost the first limitation people would hit is they'd want to change the graduations (angle, depth etc); ultimately there's no incentive for a manufacturer to develop such a technology.
That said, there is a kind of prior art in mixing sensitivity in-camera. If you look at some of the early Fuji DSLRs you'll find the
SuperCCD SR sensor which used adjacent photosites with differing sensitivity to extend the dynamic range of the sensor (at the expense of resolution.)
The colour reproduction was very good and it handled extreme lighting situations well - but ultimately in the DSLR game if you're not Canon or Nikon you often don't get a look-in and (arguably) they are one of the most underrated of the early DSLR's (and they used a Nikon mount, so there was plenty of excellent glass available).
It should be possible but may require a spefic hardware design to do so. Magic Lantern does something similar (but not the same) on standard Canon sensor with dual iso which means every other line has a different iso, reducing vertical resolution and increasing aliasing. Having differnt Iso for both halves of the senso would not negatively affect resolution but might cause problems at the border between both when you want to create a smoth transition. You would just need to be able to process each line or half of the sensor (the ladder making it probably easier to produce but harder to handle, as the horizon is fixed) differently.
Magic Lantern: https://www.magiclantern.fm/forum/index.php?topic=7402.0