Every single ambulance tasking - broken ankle through to cardiac arrest - has the potential to require critical care. Would the game support a mechanic where each individual patient has a probability of requiring HEMS (or ground based critical care) from 0.1% for a non-injury fall through to, say, 5% for cardiac arrest? FWIW, I can honestly say there isn't a single category of call that I haven't gone to.
Similarly, every job has the potential to be a Panic Button Activation. Could this be ported across to ambulance missions (perhaps with a slightly lower frequency!) requiring perhaps 1 OTL, 5 police cars, 1 DSU.