My suggestion would be to have 6 card slots, but arrange them something like this:
[A|B] ° [C]
Here slots A and B are placed directly next to each other and share one shorter rail in between. This would allow us to fit either two standard cards or one double sized card that would occupy both slots. Latter option would drastically extend possibilities and simplify card design.
Imagine these as one double sized card to get the idea.
Having so much real estate would even allow integrating a deployable antenna to the side of wireless modules.
Of course, this would require rethinking the mechanics and the latching mechanism, so it is not a trivial change.