I have had the same question as the O.P. Especially when FX themselves describe the Maverick as "A rifle purpose built from the ground up with heavier pellet and slug shooting in mind." And it is spec'd to be only 8 ft-lbs (6%) less powerful than an Impact.
I personally wonder whether FX is truly interested in big bore guns, or whether the Impact .35 was just more of an internal design exercise to them to see if they could make a big bore.
I think that this statement is right on. It is reasonably well accepted (and is my personal experience based on ownership of an M3 in .35 cal) that the Impact is a larger bore pellet rifle. As such it is not a 'true' big bore air rifle, which would typically be designed to shoot heavy slugs with power and accuracy. The Impact .35 kind of sits alone in a region between 'standard' air rifles and big bore air rifles. Probably because it is at the bleeding edge of the gun's design limitations. And at that, it already started out being more powerful than the Maverick.
So FX can say that they have a big bore rifle in the Impact .35 because it shoots pellets larger than .30 in diameter. But true big bore doesn't seem to be something FX is much interested in, so I highly doubt that they would spend the additional money and effort designing a larger caliber Maverick.