Realism

Realistic aircraft utilization

No airline has the aircraft busy 20 hours a day. For short haul and regional flights, they often only run during the day, rarely during the night. RyanAir and Southwest have utilization at about 11 hours. That's because they focus on very quick turnarounds. Larger, more quality airlines would have even lower hours. Long haul aircraft might have slightly higher hours as they will fly both during the day and during the night, but anything over 16 would still be very unrealistic. On average, I believe about 10 hours is the number you would be looking for.

Realistic seat configurations

Apart from airlines like Ryanair, who squeeze every seat they can in, most airlines, even when they operate economy only, will still have a few less seats than the very maximum. The maximum in game and given by the manufacturers is purely for safety reasons. If you want any form of legroom, you need to reduce the number of seats. Use SeatGuru and look at airlines like United at see what kind of configurations they use. Obviously in the real world, economy plus seats are used to try and make a bit more money of that extra bit of legroom, which isn't in the game yet, so I would recommend just averaging it out.

Realistic choice of aircraft

Consider the kind of aircraft that an airline of that time period and of that country would actually use. For example, an American airline is not going to be using any Russian planes. They are also more likely to use Boeing (although not definite) since Boeing is an American company. Also consider the fact that an low cost carrier such as Ryanair, Southwest etc. will often only use one family of aircraft. Ryanair strictly use only 737-800 aircraft to save on costs. Having a few of every single aircraft available is definitely not realistic.

Realistic number of aircraft

You will see some airlines in the game have thousands and thousands of aircraft. The largest airline, American, has just under 950 aircraft, with just under 650 in their regional subsidiary of American Eagle. This is obviously a lot less than the thousands used in the game, and the majority of airlines won't get close to the amount American has. Although as the game goes on, you can expand beyond these numbers, you need to be realistic about expansion too. Having hundreds of different types of aircraft all ordered at once is never going to happen in the real world.

Realistic use of aircraft

Actually use the aircraft for what they are designed to do. If it is a regional flight in the US, a 737 or an A320 series aircraft is the most you should really use. If not, you would want to be considering the Embraer and Canadair regional jets, or turboprops like the Q400. If you are running an island hopper, then use island hopper aircraft. If the daily demand is 90, don't put a A380 or 747 on once a week, put a regional jet on a couple times a day.

Realistic IFE and IFS

Spam IFS (1 star everything with max prices) is extremely unrealistic. Even Ryanair will offer a certain level of quality in their food (although not much) and they won't charge you £75 for it. Most large international airlines, especially on longer flights, will offer food, drinks, entertainment and luxuries like pillows, absolutely free of charge. Although adverts are realistic, especially in the US, having them every minute is definitely not!

The best thing to do is to look at what real world airlines actually do. Whether it be the routes they fly, the hubs and airports they use, the types and numbers of aircraft they use or the services they actually offer. It is very very unlikely that you are going to be top of the rankings by valuation if you run your airline like this, but if you want realistic, I would say these are the things you need to bear in mind.