The delays in developing the sensor was apparently a big reason for the big firmware update for the 7D.
... as theorized by many posters in the cr forum. However something "new" doesn't necessarily mean "better", it could also just be "cheaper to produce". But since Canon isn't competitive with the current 18mp version anymore I'm very curious if they'll just upgrade the mp count or can do something about the current low and/or high iso/dr limitations.
If they want to spread three cameras across 2013, the 7d2 as the most expensive to arrive first cannot be that far away.
Upgrades to the sensor will likely be lower noise for the readout, a tweak to the Bayer Filters, and more readout channels. We'd also likely see the embedded AF points as seen on the T4i. This will make live focus faster as well as enable AF during video.
Overall, there are many Sensor tweaks possible. There will be no miracle, however, the laws of physics still apply, and sensors are getting to be about as good as they can be, so its just small increments of improvement.
Of course, there are some technologies in the R&D stage that have the potential to be large improvements, but these seem to lose value once they actually try to build a working sensor.