There should be a conference between those of us who want this, in order to discuss the rules, but the basics would be those:
Use realistic cabin configurations: for modern aircraft we can use Seatguru but for older aircraft, it is complicated. We should also make a database with cabin configurations of older planes, I know a lot of websites that feature cabin seatmaps of older planes, so I can share them in a subforum, and classified by type of airplane. You are not forced to use them as they are, that means that you can adapt them to meet your airline's needs, but always exercising common sense
No soviet aircraft if you are operating from a western or western-alligned country and no western aircraft if you operate from the eastern bloc or any soviet-alligned country until 1991: I think this is pretty self-explanatory, if you operate Il-18 from France just because it's cheaper than a 727, or if you operate 737's in the USSR just because it's faster than the AN-10a you will get the boot. Exceptions can be made though, such as the case of some African countries, China, Iran, etc. I know that after the glasnost and perestroika, some airlines from the eastern bloc, such as LOT, Malév and Interflug, started operating 767, 737 and A310 respectively shortly before the fall of the Berlin wall and the end of the Soviet union. In such cases it is allowed but always exercising common sense. That also applies to modern day North Korea and Cuba, though with Cuba some exceptions can be made. Cubana has operated some western aircraft, mostly leased from European carriers and Aerogaviota, a Cuban airline that caters mostly to tourists (which I also flew), flies ATR 42 along with AN-24 and 26 and Mi-8. That leads me to...
Flights over the iron curtain aren't prohibited, but keep them to a reasonable minimum: I know that some western airlines flew to Moscow and that Aeroflot flew to some cities in the western bloc along with some other airlines situated in the eastern bloc such as Interflug. However, exercise realism. If you operate a western airline, fly only from your main hub and fly only to the capital cities of those countries and if you operate a soviet airline, you can fly only from the capital city to other capital city, exceptions can be made with St. Petersburg for example
Strictly no ordering of planes past their production end date: This is self explanatory, if you have pending deliveries of the DC-7 until 1996, you get the boot without further notice. If you really need to order a specific type of airplane past its production end, there is a grace period of no more than 2 years since the production ends
No use of old planes unless it is for realistic purposes: for example, if you operate some small airline from some inhospitable region such as Alaska, Yukon, Tatarstan, Siberia, etc, it's ok to use some old propeller aircraft like the DC-6 Convair Metropolitan, AN-26 or Yak-40 but not if you operate a major airline or flag carrier. If you use a Britannia or any similar aircraft in the 80's to have an unfair fuel advantage over other airlines in order to dramatically reduce your prices, you will be warned, if you don't change your attitude, and open more routes with that type of aircraft, that will result in your expulsion
Flooding of routes with the purpose of dramatically reducing the price of air tickets is forbidden, however, using connecting passengers to raise the number of passengers in some specific routes is permitted as long as it is realistic. For example, let's say I operate Aerolíneas Argentinas or any major carrier in Argentina, I'm part of an alliance and I have a domestic hub in Aeroparque Jorge Newbery (AEP) in Buenos Aires, so I want to open a route to Neuquén (NQN), in the route configurator of the game, it shows that only between 50 and 70 passengers fly that route while in real life, that route is flown by roughly 500 passengers daily. If I have a good traffic bonus, i would raise that level as much as I can while leaving default prices so as to meet real demand. In that case, pulling profit from connecting passengers bonus is allowed but to a reasonable amount. However, doing that in busy routes is not allowed, determining where to use connecting bonus fairly is up to each one's common sense
It's not recommended to try to fill all the seat demand on mainline, high demand routes: leave something for the rest, specially if you're playing in vintage worlds, bear in mind that demand levels are modelled after modern day figures and people didn't fly that much 50 years ago because flying was EXPENSIVE, so, don't try to push 30 707’s on JFK-LHR to cover the demand because that's unrealistic as f***, put three or four aircraft and raise the price of tickets, I recommend the competition to do the same
Maxing out aircraft utilisation is strictly forbidden: the absolute maximum is 16 daily hours. For more realism, I suggest using planes as following
Regional propeller aircraft: maximum 6 or 7 daily hours
Narrowbody jets or any aircraft doing main short haul routes: 9 hours maximum
Widebody aircraft doing medium haul routes or older equivalents (720, DC-6/7, B-377 stratocruiser, Convair 880 etc...): 12 hours maximum. This applies also if a widebody such as the A300 does a short haul route
Long haul aircraft: 16 hours maximum
This chart is more of a suggestion than a rule and it's flexible as long as the absolute maximum of 16 daily hours is respected
There may be more possible rules to suggest and I may have more, but these are the basics at least for now