Here you've got some of my suggestions;

- Different levels of difficulty

This would affect the difficulty of each mission given, how many suspects there are in a shootout and such.

- Different levels of intensiveness

How fast It goes between calls, could be useful when you'd like to patrol, or when you "roleplay" something on your own and don't wanna get spammed with calls.

- Ranks, Promotions and Demotions

A Points system that is based on your stats, a backup call would lower your points, an arrest would increase them, and If you arrest someone without any backup It would give bonus points.

- Vehicle Inventory (Trunk)

With misc police equipment, such as M4, armor, extra ammo

- On Foot patrol

Would not require a vehicle, however you could request backup of vehicles and It would have different missions or place missions closer

to your location

I like the foot patrol ideas. But i really don't think in the trunk you should be able to get M4.

I also dislike the ranking system that calling for backup lowers your points.

Good ideas.

If the rank thing is implemented, I am screwed.

Spamming backup is too fun, especially online as it really confuses people why police cars are coming out of nowhere and shooting them ;D

Well, a rank system would be fun, no matter how It's setup :P

Don't like the points/rank system - how about making it optional if it is phased in. The other suggestions are good.

Well, a rank system would be fun, no matter how It's setup :P

Yes, it does matter how it's setup.