Increase the number of players, remove all players from radar.
I play on PC, so as well as the people trying to screw you legitimately you also have the kids with mod menu's making free mode pretty much unplayable, IMO more people in the lobby will not help this.
But if the hacking issues were sorted more people and your Radar idea would be great.
I would have it so you were hidden on the radar except for certain occasion's for example,
When in very close proximity,
When you have a 3* wanted or higher. (other players can get cash for catching you) but you only show up on the map if you are in police sight.
You are shown up on the map for 5 seconds when picking up supply's and doing deliveries then hidden perhaps flashing up on the map once a minute for a few seconds.
If you phone someone like lester you can have someone traced for a short time.
Bounties only in close proximity and a flash up every 1 minute,