Why this topic doesn't have more replies? It's awesome!
This is one of the thing I think is missing to LSPDFR, it's a progression system which makes sense as a gameplay feature.
Players need an incentive to come back, make a good job (or not) and have some sort of feedback from the game, feel like their actions have an impact.
Raising ranks wouldn't make sense in the mod, however, unlocking costumes/cars/weapons and other stuffs would be interesting.
Would love to see that implemented in the game!