A long while ago these were put forward:

https://www.digitaltrends.com/mobile/smartphone-e-ink-covers/

but I can't recall that there was ever an actual product.

It seems to me that a case which:

• had an e-ink screen which would show when protecting the device (mirroring the display)

• which would open in such a way that it could either extend the display, or function as a keyboard/touch pad/stylus digitizer while supporting the cell phone

would be a nice option, esp. if some manufacturer would standardize their form factor over multiple phone generations allowing the investment to be amortized over the lifespan of several phones.