I've only recently started playing, but one thing that I think would make the game much better (by creating the possibilities for better competition) would be to break down passenger demand on each route by the type of passenger looking to fly.
This would probably tie in best with some sort of city demand based system as discussed elsewhere, because then the passengers could take in to account things like airport distance and whether flights were direct or connecting. But either way, here's how I'd do it... I'd split demand in to six groups.
Leisure $ - Backpackers and others looking for a very cheap getaway. They are willing to travel a long way to an airport and/or change flights (if feature added), plus they'll endure older aircraft with less comfortable seats if it means their flight can be cheap.
Leisure $$ - Mainly family travellers, still looking for cheapish economy tickets, but not willing to travel quite so far to board a plane and how comfortable the economy seats are/what on-board service is provided (if that's added in at some point) will also be a factor.
Leisure $$$ - Rich leisure travellers, who despite the fact they're going on holiday would like to fly business class (or premium economy if it's added), and are looking for convenience and comfort.
Business $ - Businessmen looking for a cheap deal. Have similar demands to Leisure $$ passengers, but perhaps slightly more fussy about flight days/times.
Business $$ - Businessmen looking for good quality business class travel. Don't want to have to travel too far to the airport, but would prefer a direct flight if possible. They fly a lot and so know about planes, and will turn their nose up at older aircraft somewhat. They're still mindful of price though, and will scan the different airlines for the best business class deals.
Business $$$ - CEOs and the like who want an airport on their doorstep and a first-class ticket in their hand. The sky is the limit price wise... if you can provide the right service at the time they want.
Then for each route you could show how passenger demand broke down by type. So for example currently in the Beginners gameworld I'm in demand for LHR-JFK is about 3000/day. This might break down as L$ 800, L$$ 1000, L$$$ 100, B$ 500, B$$ 500, B$$$ 100, so with a reasonably large number of business class passengers but the majority still looking for cheapish economy tickets. However, flights from London to the Spanish coast would see a much smaller percentage of business class travellers in all areas, whilst the majority would want cheap economy tickets in the L$ and L$$ categories.
What this system would allow is for different airlines to appeal to different sub-markets on the same route. So one airline might try to offer a very cheap service, whilst another would go for all business class and others in between. In my opinion it would add an extra dimension to gameplay - and this is about the most straightforward way i could think of to do it!
Apologies if a similar thread exists somewhere - I couldn't see one but that may just be because I haven't got the hang of the search feature yet