Might be interesting to use some kind of Emergency Token System distributed via chance. A "lottery" is a fair way of distributing things that have a limited quantity (and emergencies should probably be limited due to the unfair nature / impact on other aircraft.
It could work something like this:
Every member has a low chance to receive an emergency token over time (maybe a chance per flight hour or something?)
There are different types of emergency tokens that can be received, i.e. engine failure, medical emergency, loss of GPS, etc
Members can opt-in to use an emergency token on a flight when they want to simulate that emergency type
ATC is allowed to give emergency priority like the real world, since the tokens are evenly distributed.
If needed ATC could have the ability to allow or disallow emergency token use as they wish.
Pros:
Allows prioritizing emergency like the real world
The most experienced pilots have more of a chance to get a token, so more likely to be realistic
The token drop chance allows network designers to control the impact of the system
Cons:
Creating a token system for emergencies doesn't really match the real world. However, this is a compromise and would add realism without getting out of control.
