IMHO there are 2 possible - quite obvious - things that Amazon could do:
- a device with 8" screen or wider;
- they could experiment a colours device.
But, especially for colours, I don't think that it will come so soon, it will come when they are sure that it can be a really good device.
About larger device if Amazon produced Kindle DX some Years ago they could do a similar modern Kindle (but until now there isn't anything like that).