For one of the hits you can make it all assasin/sniper style.
eg:
the person who hired you wants you to assasinate {blank} companys owner/president. The money/gold/items you are offord is amazing. You're in.
At the beginning of the mission, you need to kill someone to get there sniper rifle. But that isn't so easy because they're a cop. All you have with you is a knife/katana/iron knucles/nothing. Once that cop is dead you have the sniper rifle for the job.
Now, you need to go dig up all of the info you can about the company owner/president. You need age, name, phone number, adress, hobbys, previous wifes/husbands, were he/she goes after work, and everything else you can get. You go all over town, searching bars, garbage cans, and "victims (hehehe.)"
Now, you need to get a coverup name. You go with the last name of the cop that you killed. As for your coverup last name, you go with Smith. Your name for now is {blank} Smith. You need more than a name though, now to get some clothes. You go to the stores to get some clothes, and a fake rubber hand, with new shoes.
You need a getaway vehicle, and a driver. What better than a helicopter. For the driver, you "hire" one of the best helicopter pilots in the province/state. He knows how to do everything, so you kidnap his/her family to be sure he/she doesn't pull anything funny.
Just to find a good vantage point. For the vantage point, you go on a building a few miles (or were I live, kilometres,) away. You find the building, but no vantage. So you kill the window cleaner, and use the weird pull elevators. Now, you can see the victim through your scope easily. He is sitting in his office. He takes a sip of his cofee and stretches... boom, he's deceased.
The cops are after you, obviously. You have to wait for the pilot, and soon he/she arrives with the helicopter. But he/she doesn't fly you home, instead right into the ocean. Luckily you had bombs around his/her family. BOOM, they're dead, and the pilot died in the crash. Now you swim back to shore, HiJack a car, and get your prize.
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This is just an example. I am sure that if you put this in with all the ideas you probably have. I think my idea alone could be a whole map! Good luck with making this, and be sure not to mess it up.
How about first you were select and kidnappened by a gang, they somehow notice your accurcy skills and train you to be their hitman. You train by killing and doing a few task for upgrade and vechiles. When you want to leave, you kill their boss and the police were looking for you to become a mercarny.
I suggest you do a installion with 128*128.
After a while you join another gang. Thats all.