As stated before, this is how it used to work - and it didn't work very well.
As far as I've remembered, previously the whole town income was based on trade. It didn't work because players still prefered the REALLY safe cities. Now it would be just a bonus. If (accidently or thanks to effort) some city became so safe that that ANYBODY will decide to trade in there, gang in control will get their cut.
Anything based on players killing some people and leaving others alone is just doomed.
Hm... FORCING them to do so - yes, of course, but...
Players refuse to play exactly as you want them to and will do what is best for themselves.
...but making it
profitable. It will just be an another way of raising funds.
Keeping PKs away from one's city wouldn't be a must. Nobody will force anybody to do so. Just as the whole fight for control thing isn't obligatory and many gangs (including mine) just don't go for it. But still many do - because it is fun or a challange or simply because it is profitable.
The whole point of my suggestion is to make keeping peace in a city profitable. It is simply a matter of mingling with numbers to make it that even with 100 deaths a day gang will get similar funds than before.
But let them know that if some of them will stop killing other players in their own town, they will get more money
But let them know that if they will stumble across a PK in their city, wasting him will help them in getting more money
But let them know that if they will keep an eye on what is going on in their city, they will get more money.
Because money is a wonderful thing. People do crazy things for money, also in Fonline. They go through tedious process of crafting dozens of unneeded weapons to sel them for money.
They clean brahmin crap for hours for money
They hire themselves as a ore providers for gangs and spend hours on digging that goddamn ore only for money
That is why I sincerely believe that if we give them another opportunity to earn money, SOME of them will go for it. Even if it will be such an irritating, meaningless, tedious and idiotic task of guarding a city they are supposed to guard.