A fully transparent LCD screen would be layered over an E-ink screen. The LCD would provide a color display that can change rapidly for animations, video, games, etc. The E-ink would provide a low power, eye friendly display for text and other visuals not needing the LCD capabilities. The best of both worlds.
When the LCD would be off, it would be fully transparent and appear just as a glass overlay of the E-ink screen. Front-lighting for the E-ink would be provided through LED lights at the edges shooting through the LCD pane. It would thus appear and operate like a normal front-lit E-ink device, other than the “glass” might be a bit thicker and may also be darker, requiring more front light.
When the LCD is on, the front-light will be used to reflect off the white of the E-ink screen to light the LCD. Bright external lighting should also be able to help and allow turning down or off the front-light. The LCD might make use of the E-ink behind it for added contrast, darker darks.
Ideally, this will also be a touch screen with active digitizer, and rugged enough for portable displays.