Not really sure if this is all that great of an idea, but I thought it'd spice things up a bit and also enable players and developers to earn some spare cash, keep in mind that all of this is subjective to change and simply a thought.
Basically, in the city, there would be a place to fight, the only way to enter these fights would be by using some kind of added currency, this new currency would only be found in donator packs or bought from the developers themselves.
players could then use these coins to host or enter fights, and whoever won the fights would win 3/4s of all the tokens put in by all the entrants of said fight. all other entrants would lose the entry fee.
developers would then buy back these tokens from the players with real cash.
the amount of money one token would be worth could easily change (I'd recommend one cent per token, if its too little people will simply up the entry fee, but it cant possibly be too much), but as long as its set up using these basic rules, no matter what is done, the developers would be unable to lose any money, and would effectively gain money from every fight taken (the money paid for the 1/4 tokens lost in every fight were paid for by the players, but the devs dont have to buy them back if someone wants to sell them)
Whoever hosts the fight, essentially acts as a defender, and the player joining, an attacker
this would probably be set up for mostly 1v1 fights, but maybe team fights could be feasible as well
An example;
Person1 hosts fight, entry fee; 200 tokens
Person3 joins,
Person1 wins, taking home 300 tokens out of the paid 400
Another Example
Person1 Hosts a 2 v 2 fight with an entry fee of 200 tokens
Person2 joins Team1
Person3 and Person4 Joins Team2
Team2 Wins, Person4 and Person3 Each earn 300 tokens
inorder for game balance issues, perhaps Fighting classes are set up so that those with extremely high stats dont overpower all the other players, i.e
Class A Billion+ total stats
Class B 1 Million-1 Billion total stats
Class C 100k-1 Million total stats
Class D 50k-100k total stats
Class E 1k-50k total stats
Class F 1k or less total stats
Using this system I cant really imagine anyway it could possibly alter economy other than boosting the demand for donator crates and temp weapons