A lot of the larger, better known airports from real life (LHR/MCO/CDG/HKG/BOS/JFK/ATL/ORD/NRT/FRA/FCO/LAX/SYD, just to name a few) are likely to be very crowded in the game, and in effect, be very hard to operate from due to the amount of players in that market (not to add those from other cities having routes from there.) Typically, you'd be lucky to get enough DOP to cover your monthly costs in such a situation. What you should do is look for markets that are smaller than those, like if you want to do something out of Orlando area (I'm using it as an example since that is the airspace I am most familiar with), instead of looking at MCO, why not look at the secondary up in Sanford, SFB? Typically these smaller airports will go unserved, despite them having what I find to be reasonable passenger numbers.