Not sure if it's been done before but I haven't found anything for LSPDFR. Not sure if it's possible but just thought it would be a good mod if someone could make it.
The general concept is the ability to level up through LSPDFR actions such as arresting people & completing callouts etc.
This could be done with a virtual currency to purchase equipment, backup etc or all through exp points. You could be rewarded with xp or the virtual currency for doing good things and penalised for doing things wrong such as civilian casualties / deaths.
I figured you could level up once you have more xp points which would result in unlocking better weapons and cars (like a promotion essentially). Alternatively you could spend virtual currency on a promotion to earn better gear or even unlock better callouts - that might be a bit farfetched for the mod though