Here's my guess:
Air line gets merged into one 12" model.
New clamshell design, moving ports to the back of the machine. This gets rid of the "stethoscope" effect with a Thunderbolt display and may make it easier to package the internals of the machine. This also would allow them to go for a more aggressive taper or thinness, as the machine is no longer bound by the height of USB/Thunderbolt.
IGZO higher resolution display, which is probably a foregone conclusion at this point. The rumored resolution of 2304x1440 is puzzling, given there are other higher resolution panels of similar size in production now. A new method of UI scaling could make this possible, or the resolution could be completely different from the rumors. 2560x1600 in a 12" form factor would be awesome...
Broadwell Y series processors - under 10w TDP, fanless design. Getting rid of the fan helps with making it thinner as well. Given another significant boost in consumption from Broadwell, plus moving to a Y series processor, it seems possible to have 10 hours in this new ultra thin form factor, with processing power equal to the 2013 model.
Its a given it'll have PCIe SSDs, AC WiFi, upgraded FaceTime camera, Thunderbolt 2. A 16GB RAM option would be nice.
2lbs in weight, perfect size form factor (slightly taller to give just a bit more palm rest area), 1080p webcam with the dual mics from this year's model, ultrafast I/O, dead silent, support for a 4k Thunderbolt display. I'm hoping this thing lives up to (my) expectations
.