Add button as part of a custom carousel menu

Question

A client wants a web app built where the user can add items to a 3*2 carousel. There would be arrow buttons on either side that make it slide to the next 6 items, and so on. However the design calls for the "add new" button to be one of the carousel items (i.e. "fill this next empty space). That obviously means that there cannot be a 6th content item since there'd be no space for the add button.

The current design has the bottom right object appear at the bottom of every full (5 items) page, but the problem I see there is that even if there are arrows present, the fact that it isn't a full 6 will make the user where the other stuff went. It seems unnatural. Is there any other way of doing this aside from an add button that is separate from the carousel?

With 4 items

With 5 items

Answer 1

Put the Add button only on the last slide, perhaps?

As I see it, your metaphor "Fill this empty space" is at the same time mistakingly cuing that the existing items run empty already at the first page.

^{download bmml source – Wireframes created with Balsamiq Mockups}

You could also add the Add Item control to the control area of the carousel, instead of in the item area:

^{download bmml source}