I think that the 6 inch black and white eInk reader is a classic design not in much need of improvement.
In ten or twenty years,
it should be possible to make them so they never need battery charging, even if mostly kept in the dark. That will be nice. Lower prices combined with more durability (gorilla glass?) would be undoubted pluses. But I don't think improvements are urgent. And I think that the device changes we are likely to see will be be more helpful in making purchases from the like of Amazon than in actually improving the experience of reading long texts.