I don't know how they would fit a new model in between the CX-5 and CX-9. Although the CX-9 seems a lot bigger from the outside, it's not that much bigger on the inside. The driver and front passenger have about the same amount of room as in the CX-5. The second row seat is only about 2 inches wider, and has the same legroom and headroom as the CX-5. If you're trying to fit three children in the second row, the extra width may be appreciated. But otherwise you probably wouldn't notice. The cargo area is slightly wider at its widest point, but the wheel housings encroach more, so the usable width for large items is actually a bit less than in the CX-5. The load floor is pretty high, so the height of the cargo area is also less than in the CX-5. With the third row folded, the length of the cargo area is nearly a foot longer than CX-5, which is the only place I can think of where you see a real benefit from the larger body.
I think Mazda made a design mistake with the CX-9. It's too compact inside for most buyers who need a third row, and too compact inside considering the exterior dimensions.
A lot of other companies have found a way and a strong market for a larger 2-row...
Escape and Edge
CRV and Passport
Rogue and Murano