Prototypes for GM's next-gen HD trucks have been spotted for the first time in both 2500 and 3500 form. 
Based on the LED signatures visible the mules are presumed to be Silverado HD variants. Compared to the current truck, the new HD will receive a positively massive grille.

Also expect many of the Silverado 1500's trick innovations to make the jump to the larger trucks, like the new air curtains that divert airflow through the front bumper, instead of around its face.  

Other shared innovations that should show up on the heavy trucks include the size, stamping, and packaging innovations seen on the 1500, which will net more interior space and more useable space in the bed. It's unlikely we see the HD debut until sometime next year.