With the new medical update, you can currently receive calls that require Critical Care Teams and separate calls that need HEMS to complete. Rather than have separate missions for each separate unit, why can't Critical Care Units also incorporate HEMS missions.
From my beliefs, CCT vehicles perform the same role as HEMS teams. The only difference would be the speed in which the team can arrive on scene and then the speed in which the patient can be transported to hospital. This however can be left down to the player to decide.
A negative towards this would make having HEMS useless when you could just buy a tonn of CCT's instead of a single HEMS. The only current benefit to HEMS is the speed/distance is can travel to get to the scene and a slight credit increase. Rather than have specific HEMS missions, this could be replaced with a larger more considerable HEMS bonus.