This may have all ready been mentioned, but I didn't see it.
I'd like to see a world with a restriction on the number of destinations an airline could have. With a limit of around 150 destinations, there are enough routes to be flown that an airline could take the time to focus on being more competitive on certain routes as well as introducing the concept of planning where an airline wants to fly instead of flying EVERYWHERE.
That would help regional airlines have a better chance of growing more without being squashed out of most markets. It would also allow those who want to simulate real airlines have more realistic opportunities.