A release in March would be the best move for Apple. The Windows laptop market has made some big moves over the past couple years and the current Macbook Pro models are starting to fall behind. Most importantly, Microsofts new Surface Book laptop was a direct attack on the Macbook Pro, so it would be nice to see Apple bring something than can blow it out of the water.
A slightly slimmer and lighter body is almost a given for the new model. Although I wouldn't expect it to be dramatically slim. The top 3 most important things Apple needs to do is:
1. Slim the bezels or bump the screens to 14" and 16" (while keeping the body the same size). The PC market is moving towards slimmer bezels and it would be nice to see Apple follow this trend. If they don't do this, they risk the new models looking a little dated next to their PC competitors. Also, as a few people have already said, bumping up to 14" for the base model would help separate the Macbook Pro's from the Macbook and Air.
2. Support DCI-P3 color gamut. We've already seen them do this with the new iMac 4K and 5K models so I don't see any reason why they wouldn't put it in the new Macbook Pro's. For anyone that doesn't know, DCI-P3 is the same color gamut that you get at movie theaters and is much wider than the old Rec. 709 color gamut that most TVs and laptop screens use (for example: the shade of red that Coke uses is not supported by the Rec. 709 gamut). Currently, the TV industry is going through a major transition by adding more color, more pixels and HDR. The new UHD BluRay format coming next year will have all 3 (4K resolution, DCI color and HDR). Sense the industry is quickly moving toward using wider color gamut (both in displays and in content) it would be very nice to see Apple support this upgrade, especially for photographers and graphic artist.
3. Major Graphics and CPU upgrade for both the 13" and 15" models. I'm sure this is a given sense Skylake will no doubt be included but its very important that we see a power bump. The next Macbook Pro's should be able to hold their own on the Benchmark test.
Beyond that, new colors, IGZO screen, 256gb starting memory and other small details are just icing on the cake. This will be a very important launch for Apple.