There will be a new Modern Times scenario starting this Friday March 4. Since you are active in the current Modern Times 3 scenario, use the opportunity to start planning for your next airline. Start researching the airports that interest you by doing route planning from them in order to determine demand on various routes, etc... (since the new scenario will be about the same time period you can expect route demand to be similar to the current MT3). Research which aircraft would be best to serve the routes you want to fly (try to choose one a/c fleet type that will efficiently fly all the routes you intend to service for at least the first 10-15 a/c ... fleet commonality is critical, especially for a start-up). Once you have an idea where you will base, what the routes look like and what a/c you will use to service them it's just a matter of waiting for the scenario to launch. DO try to join the scenario the day it launches (used a/c will be plentiful and refreshed frequently). And DO remember that the first game-turn day does last a full 24-hours (delayed first game turn) so there is no rush to get yourself organized at launch.
Also, the links that Vienair provided are a wealth of info so be sure to spend some time there.
Some quick pointers: Large a/c (such as B737) require at least 3 routes per day, 7 days per week to be sufficiently profitable; flying a route w/600 pax demand five times per day with a B737 will yield higher load factors, better profits and discourage competitors better than flying the same route twice per day with an A300; Use a/c for what they were intended - i.e., don't try to use a B747 on a 300 pax demand 1000nm route (these jumbo a/c were mostly intended for high-demand and/or long-haul international routes where higher ticket prices compensate for the long turn-around times and lack of flight frequency); when choosing your start-up a/c try to choose one that is fairly plentiful on the used market and is also still in current production so you can order new planes when the used market dries up and/or you want to replace aging a/c.
Best of luck!