To be honest I think the most likely case will be that they do the same thing they did with the iPad Air/rMini. I don't remember specifically because they've tweaked all the cameras so much without having complete feature transitions. What I mean by that is they don't always "Put the iPhone 5 camera in the new iPad Air", sometimes they "Improve the iPad 4 camera by having better low-light capabilities" which means they didn't completely replace the sensor with the iPhone 5's, they instead just updated a part of it, but are keeping the old sensor.
Anyway, I just looked up all the details since I wasn't 100% sure. The new iPhone 6/6+ cameras are the same as the 5/5s. The only difference is the front facing camera is now f/2.2 aperture instead of 2.4 (slightly better low-light capability) and the rear camera has focus pixels now which makes for faster focusing. Facetime camera is still 1.2MP, and rear is still 8MP.
Current iPad Air/iPad Mini have f/2.4 aperture rear cameras (5/5s feature) with older 5MP sensor. The front facing camera is unlisted with aperture, so I'm guessing it's the very old f/2.6. Both have 1.2MP front facing cameras, the same resolution as the iPhone 5/5s/6/6+. Given this information I would suggest that the new iPads would likely be given f/2.4 aperture for the front facing cameras, and then perhaps f/2.2 for the rear. Potentially a bump to 8MP but I don't think so. I definitely don't think they'll get optical image stabilization, that would make the camera jut out (as on the iPhone's), and also cost them way more money which would raise the price of the iPad Air (possible, but stupid).