I came up with this yesterday, and obviously don't work for Rockstar so I'm probably way off, but what if the hidden packages in GTA V are a series of disposable cellphones randomly scattered about the map, which, when found and answered, give you a challenge that then unlocks a reward? This would be more realistic than any of the previous hidden package types, and also much more fun.
For example: you find the first one sitting on a workbench in an empty lot (they'd obviously have a unique ringtone that you'd hear when close by), so you walk over and press the action button to interact (pick it up and answer). Disguised voice on the other end says "Congratulations, this is your lucky day, if you got the balls for it. I need stuff done, and I reward those who help me handsomely. There's a guy over in <neighborhood> who has been causing my associates some discomfort. Go and cause him a lot of discomfort. His picture is attached". The protagonist then puts the cellphone in his pocket, and you go where the guy should be and beat him up. Shortly thereafter, a van pulls up and tosses a duffel bag out close to you, containing money or a powerful/unique weapon, weapon upgrade, or keys to a vehicle. You then dispose of the cellphone after collecting your reward.
Like I said, pure speculation, but tell me that wouldn't be a lot of fun, while still being significantly more realistic than any of the previous hidden package types. Imagine a 100 or more such phones scattered around the map