Just thought of this idea when considering the drawbacks to my last suggestion (and many others), which are usually something along the lines of "it helps big airlines and hurts small airlines."
You have two tiers of games: One tier would be similar to now, with certain "realism" features turned off (thereby making it easier for smaller airlines to get started), and another tier would be more of an expert mode, where players are expected to fend for themselves in an environment with more realistic passenger preferences and flight economics, meaning the airline that buys a 747 and flies it NY-Tokyo can make tons of money, while the airline that tries competing JFK-SFO with an ATR42 vs. other airlines' jets gets its butt kicked?
The tiers would be set at the start of the world, and shown in the world description, so that players would know what they're getting into before joining. Those that just want to play a leisurely game can join the easier world, and those that want to be extra competitive and risk bankruptcy if they can't compete could join the tougher world.