Given the information in this article and other things I've read, this is what I think they might do:
iPad Air 3 with Force Touch. Aesthetically exactly the same. A9-series chip with three cores like the A8X but a little faster. Possibly a better quality 8MP camera like recent iPhone models.
iPad Mini 4 gets Force Touch and an A9-series chip with two cores like the A8 but a little faster, 2GB of RAM and the 8MP camera in the iPad Air 2.
The iPad Pro gets Force Touch, A9X-series chip with four cores, maybe 20% faster clock than the iPad Air 3, 4GB of RAM, and official stylus support (pressure, maybe tilt) for digital painters, designers and photographers. Might have USB-C for additional limited accessory support, much like the iPad has limited accessory support for downloading external camera photos.
iPhone 6S gets an A9 chip similar to the iPad Mini 4, but maybe clocked slightly lower, along with Force Touch, faster LTE and an upgraded camera system and 4K video recording.
The iPhone 6S Plus will pretty much have the same exact chip as the iPad Mini 4 along with Force Touch, faster LTE and an upgraded camera system and 4K video recording. Not sure what the differentiators might be on the Plus at this time. Perhaps it will remain the only iPhone line with optical image stabilization? Though both lines are rumored to have the same 7.1mm thickness this time, which means they should be able to put it into both. But will they?
I think all new models will get an upgraded Touch ID sensor that recognizes fingerprints faster. Furthermore, all models are likely to see their color options tweaked like we saw from the iPhone 5 to the 5S and iPad Mini to Mini 2. Possibly tweaked to be more in-line with the Apple Watch. I think nearly all models will get an upgraded 1080p FaceTime camera. Nearly all models will see improved battery life.
All models really need a bump in base storage capacity to 32GB. However it's probably unlikely they would do that until 128GB chips are cheaper because they probably want to retain that big up-sell. They'll probably wait until they can do 32/128/256GB for a decent price, and that will probably be possible in autumn 2016. However they may add a 256GB iPad option, just as they used to have 16/32/64/128GB options before switching to 16/64/128GB last year.
This all being said, many of these changes aren't too exciting. I would be excited about the iPad Pro if it turned out as I've said here, however I would probably wait a year until there is more pro-level software available that can really take advantage of it. It's a shame Apple killed off Aperture because something like that on the iPad Pro would have been killer. Anyway, the iPad needs something more to take it to the next level and get people excited about it. It has kind of stagnated in recent years. Personally for me, going from the iPad Mini 2 to the iPad Air 2 was a big jump because of the performance increases. But the extra RAM just made it more stable and retain apps and tabs in memory. It got the iPad back up to par. Now what are they going to do for normal users? Split screen software features are nice but I hope they really embrace the possibilities with Force Touch UI to take iOS to the next level and make it a true productivity powerhouse. I think it's very possible we may someday look back on Force Touch as a landmark moment, possibly up there with the keyboard, the mouse, the click-wheel and capacitive touch.