I think most of the players would enjoy changing things up a bit by introducing some additional measures to help larger planes so we can stop doing some these things: e.g. B767 / A330 for LH, B757PF for cargo, med props / jets for slot grabbing and SH freq spam
Some suggestions:
1) Tune up "too small" penalties: They should also apply to VL planes like B767 and A330 for high demand VLH routes, likewise for B757PF
2) Increase slot costs exponentially or make them a higher running expense
3) Bans: Large slot constrained airports will ban small and med planes or demand high fees for service
4) Demand bonus for large planes: e.g. perceived safer, more spacious, less noise, prestigious
5) Increase demand variance: Demand has very high variance (due to time of day, season, holidays, fairs, etc) - therefore larger planes will come out better on average by capturing more residual demand