I'm pretty sure that the e-ink displays aren't remotely capable of it, but a fold-able display would be nice. For a larger format application, unfold it to 8.5x11 and for normal ebooks, a 2-page bifold like holding a normal book.
The 2 page bifold would be worth it even if you didn't need an 8.5x11 page, just to be able to hold your ebook reader like a paperback.
ED
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