Combining a few of the suggestions, possibly adding into a super-realistic mode:
- Randomized go arounds (that increase in bad weather or wind)
- Situational go arounds (if a departing plane isn’t far enough down the runway)
- Departure delays for wake turbulance
- Either sequencing based on departure track or TRACON departure delay based on aircraft destination - you could also consider pre-scheduling preferred arrival runways)
- At larger airports, planes moving back and forth to maintenance yards (or being tugged in from maintenance to a gate)
- A plane not ready for takeoff
- FOD sweeps where runways are shut down for 10 minutes
- Ability to open / close certain taxiways and runways
- Emergency scenarios
- VIP movement (a ground stop no movement allowed for X minutes, no arrival clearance
- Re-establish arrival (immediate go-around from arrival sequence and just turn back to base and jump in again)
- Better visualization of airport-specifc STARS - my home airport (KRDU), most approaches pass pretty close parallel to the airport before they turn base, would love to see those visualized. I know the NY airports have some interesting ones as well, LAX/ATL are actually pretty boring in that regard.