I expect a new redesign because:
- usb-c ports need to be included
- Innovations from the rMB will come to the MBP
- It has been 4 years since the last redesign (previous model lasted from 2008 - 2012)
- I believe they will be able to make especially the 15" a lot thinner/smaller because I don't believe the new MBP will have a dedicated GPU with the performance that the Iris Pro 580 is delivering
The new redesign will come in March or June. I hope for March but as long as we don't have any concrete rumours it is safe to bet on June. But we still have some time for leaks/rumors. Last year the rMB was leaked only 1 month in advance. So the next 2 - 3 weeks will be crucial is the March event is about to take place.
The MacBook Air won't receive a redesign. It will eventually be replaced with the rMB so expect a speed bump.
I know a lot of people think the rMB will replace the Macbook Air, but I don't know that I agree. The performance gap between the 4.5 Watt Core M's and the 28/45 Watt Macbook Pro CPU's is very large. It seems weird to me that they would drop the 15Watt line.