I think the next iteration is (or should be) a Dash button with an e-ink screen built in that can order any item and the screen changes to display the brand and name of the item it's setup to order.

Trying to add a changing ad screen to this would kill the battery life because of the check-ins to fetch the next ad. I also don't see that generating many sales because the customer would still need to change their set order, I think few people would accept Amazon outright changing what the button ordered based on the current ad.