Or, better still, how about just "MacBook"? Something between the smaller, lighter 13" Air, but not so full-tilt "Pro". Just a simple, mid-range MacBook...more than Air, less than Pro. And priced accordingly ($1,499 or so?).
I remember that short-lived 14" iBook years ago, but I don't think it did as well as the little default 12". It was just blown up in size, but I don't even think the resolution was increased, so it was as though it was hit with a growth ray, and scooted up about 15-20% in size. I don't think it sold all that well, and most people don't even remember it.
I think calling a 15" model "Air" would be odd. It's automatically going to be larger/heavier, just from its screen size, than anything we've ever known as a MacBook Air. So I'd leave off such descriptors and just have it be the mid-range notebook for the mid-range crowd, with a larger display.
But you're right: it could/should replace that odd-duck 13" MacBook Pro that's still hanging around. It just doesn't need to further muddy the waters by being called "Pro" (or "Air"), IMO. Make it its own thing...it'll be a size (15") currently not used by any other MacBook.
Maybe it gets a few more BTO options (this or that core count, etc.).
Whatever it winds up being, one thing I'm certain: it'll still be 8GB RAM and 256GB SSD (the single kind too), at its base price.