When Apple made the retina versions of the 13" and 15" MacBooks, they shrunk 'em down and chopped off ports. To me, that's pretty much what the new MacBook is.
I don't see how they could put a retina display in the current Air and maintain the battery life. On that note, I don't see them coming out with a new Air that has less battery life.
To fix battery life, they could add more battery (and make the Air bigger and heavier), but I don't see them doing that, either.
I don't know... the air lineup is pretty bulky, contrary to popular perception (including mine). Its thickest point isn't that thin and the 13' MBA is bigger in the other two dimensions than the MBP lineup (!).
Now consider
1) efficiency improvements in the CPU (and its integrated GPU).
2) More space with new keyboard (less travel, butterfly switches)
3) More space with new trackpad (no actual click or travel, but magnets)
4) Improvements in battery tech
It wouldn't be any bigger. You just take the size improvements from the new Macbook, but use them for extra battery instead of extra thinness, and keep the bigger form factor of the air 13' over literally every other current non-15' Macbook.
I'd say they should get at least say 90% of their battery life. Maybe not 12h, but 11h instead. But retina, and with a new CPU, with 2 USB-C ports.
Do I think it'll happen? Absolutely not. I was really surprised they didn't name the 12' an 'Air', but when they did, I pretty much thought that the air was doomed. There's no way they're going to come out with an Air next year that's thinner and lighter than the new Macbook (only way to let the 'Air' name make sense), yet isn't a gigantic step back in terms of performance and battery from the current Air lineup.
And making a 13' Retina MBA with the new trackpad, keyboard, retina and usb-c, would kill the 12' in its tracks. It just wouldn't make much sense. Hell it'd be way too close to the rMBP lineup, too.
I wouldn't be surprised if we'd end up with a hugely simplified lineup. A 12' Macbook, a 14' Macbook and a 14' MBP. The 12" and 14" were never very feasible sizes, but now that we have full-width keyboards and thinning bezels, it's possible to fit 12" and 14" into the same space of the previous 11"/13". Might even see a 16" MBP the same size of the current 15". These numbers sound pretty silly and crazy, but 12" wasn't ever popular either, until bezels got so thin it makes a lot of sense to fit 12" screens in a smaller space.
Meanwhile the Air wouldn't get updated in 2016 when the new lineup launches, would die out by 2017. Without a retina, new touchpad, thick bezels, support would shift to the MB and MBP. As the MB gets CPU updates it'll also get cheaper, while sustaining an 8gb/256gb base model that will become cheaper than that config on the MBA, leaving the MBA without updates, retina, touchpad, thinness and great value. While the latter MBP will get big performance improvements leaving the MBA wanting for serious work.
The MBA has gotten a bit too close to the MBP lately, used as a med-level developer/photo-editor machine, blurring the line between consumer and professional and providing too much value. A low-power Intel Core M for consumer-users and a heavy duty MBP for professional users is much better fit. The former is extremely portable, the latter is extremely powerful for a laptop form factor. You can optimize the hell out of those two objectives and really compete with competitor laptops in those two segments.
Anyway just wild speculation of course
Really though, once you add one more USB-C port to the MB, once every monitor for sale uses USB-C, once the MB gets a $100-200 price drop (MBA in 2007 was $1800 for example!), once the MB gets a 14' version the same dimensions as the MBA 13', with continuous updates to the Intel Core M to improve performance and battery life... there's just no way people will continue to buy a MBA.