Have you considered MIP displays?(1) Memory-in-pixel displays are commonly used on watches, wearables and they can display up to 64 colors in theory. They have high-contrast even in bright ambient conditions.
The largest one I can find in production is only 4 inches,(2) but a tablet-sized one could be great as a display. They don't use much power (just like e-ink) as a MIP watch can last 11+ months without a charge.
AKA transflective, AKA Memory LCD. It neither looks as good as the ipad, nor does it use no power (at idle) or read as easily as the kindle. It still has to use a little power, but it has the advantage of the fast refresh of an LCD, and a million times better than the ipad in the sun
As long as you only want to use it in bright light. I almost always need to use my Garmin watch's backlight indoors since it's just not bright enough to see otherwise.
And while the display is great outdoors, the colors are muted compared to what you'd see with an OLED or traditional LCD display.
I use my tablet indoors much more than outdoors (and it cranks up the display brightness to be usable in pretty much any light other than full direct sunlight, and I rarely sit in the direct while using it), so I don't think I'd want an MIP display on a tablet.