Return on investments - pretty straightforward;
-Shipping Companies
-Storage Companies (People with housing that cant fit any more shit, can use storage facilities (I.E Think Kennards) - so if you own a large warehouse area, people store shit in your warehouse, and u charge them a premium on it
-Private Security HQ Companies (Place and set-up for people to store and stash large quantities of company-owned gear)
-Real Estate (Place for people to purchase , sell, or offer rental on properties they own)
-Pick-Up / Towie Companies
-Mechanic / Autotune Service (Place people can bring cars to install upgrades for speed, steering, armour, etc)
-Shooting Range / Firearms Trainer (This is an excellent idea, because it can provide people a safe, comfortable area to provide weapons training and handling for a fee to those who want it. New ARMA 3 gamers are always starting out, and buy guns, but don't know how to use them properly. Experienced vets of the game could very easily teach others the best way to shoot and handle their guns and make use of cover , distance, etc)
Obviously some of these ideas would require additional scripting, but they're pretty sound. There are also a lot of ideas out there that would require no scripting, no buildings - only creativity and imagination. Tbh, it's quite sad that as soon as people get a fair amount of money, the first thing they default to doing is crime and shooting up civs / cops for the lolz. Yes, it's a FPS game, but srsly, if you want to kill things that badly, why not play on a wasteland server?
This is a role-play server guys.
Learn to role-play.