Obviously there are a lot of improvements that could be made to the accounting system, but this one is easy.
When it is time for the taxman to take your money, I'd like the game engine to look at the entire year versus just the more recent month/quarter to determine how much is owed. The reason for this being that the taxman will often get to the profits before a player can. For example, in DOTM we are 9 months in to the year 1986. My airline has $1.9 billion in revenue and my expenses are higher than my revenues by about $25 million as I type this. Despite being in the red $25 million for the year, I just paid $4 million to the taxman and have paid nearly $20 million in taxes despite having a net loss on the books. Last year I got a $40 million tax return, but only because I took out loans to spend the money that was taken from me.
Right now, the only way to have zero tax liability is to borrow money (or HQ in Dubai lol). If you have $3 million in profit and the tax man takes $1 million, leaving you with $2 million, then the only possible way to get that $1 million back is to spend that $1 million you no longer have, which often means taking a loan.
I know what everyone is thinking--boo hoo he paid $40 million in tax and had to take a loan to spend the money to get a $40 million tax return. I guess I am technically a "large airline" in DOTM, but really this affects small airlines far more than big ones. If you are a small airline, you will likely only have a few hundred thousand/week in profit. It can take months to save up enough money just to schedule one aircraft and buy slots. If you go a full quarter with $200k in profit, that is $2.6 million in cash. However, before you can spend that $2.6 million you'll be hit by the taxman and be left with just ~$1.8 million. The only way to get that $800k in taxes back is to spend that $800k you no longer have, despite the fact you are likely in the red for the year (especially year one). On top of this, if you have 4 aircraft that is one C-check every quarter (an airline likely has more than 4 aircraft). This means another hit of several hundred thousand dollars plus taking the plane out of the air for at least a week, hurting your profits. Thus, it because a vicious cycle where it feels like the cards are stacked against you when you're trying to get a small airline off the ground.
So in a nutshell, before the taxman calculates his take, I'd like him to look at the entire year's revenue/expenses first. If the airline is in the red for the year, take nothing. If they are green, adjust the amount taken by the taxman to be in line with the actual tax liability for the entire year versus just the most recent period. The only time a player would get a tax return then is if taxes were taken early in the year, the airline then goes into the red for the year and finished the year red, then they would get the refund of taxes taken early in the year. Instead of these ridiculous monster returns of $40+ million, they'll be more in line with the real world.