An ingame scoreboard which counts arrests depending on the spawn used.
To simplify it:
SWAT member makes an arrest while spawned SWAT Officer, 1 point is added to SWAT on the scoreboard.
This is likely to encourage competition between squads as well as increase activity in general.
You wouldn't want to see that your squad as a whole made 5 arrests in a month time!
Additionally the scoreboard can be made visible only to players using a police spawn.
Naturally something similar could be done for gangs, however this specific suggestion targets squads.