my brother and i once envisioned a game called Freelance where you could pretty much do anythng inside one massively interactive world. the name came from the idea that you start as a poor freelance photographer with a crummy disposable camera and a dream to rise to the top. by being at the right places at the right time, you could capture great action photos of car accidents, train wrecks, fires, gunfights, etc. and basically you could create any of these accidents (i.e. oil slick in the road leads to car crash which leads to massive fire) and be the first photographer there. as you gain money, you cna buy new equipment, including video cameras, cars, zoom lenses, remote cameras (throw it high up on the corner of an intersection where you know you're gang is going to steal a silicon shipment, then collect the footage later)
the cool part is that you could live a straight edge life as a reporter/photographer, buy a motorcycle and join a paparazzi motorcade, sneak up on celebrities' homes(ala splinter cell) for the perfect sunbathing shot. Or you could get involved with organized crime and milk your money off of footage of their heists (which you could no-doubt take part in).
and if you think photography is fagnacious, you can quit your job at any time... but then you're unemployed and will need to make some money, so you might want to approach a mob boss and see what he needs. Hitmen can make a good living in the big city, but you need to plan your attacks properly. You can't always walk up and stab somebody (especially in his own home; too much blood on the carpet). sometimes you need to run a little recon to see if you can sneak into his home and just snap his neck, or maybe hide in his garage until he comes home from work and kill him before he gets in the house, then use his car to drive to the city park and hide his body in the woods. Or maybe a local pig farmer would be willing to help you out.
there are countless options and directions. you can be a hitman, cab driver, cop, delivery truck driver (careful not to get robbed), or just your average joe-criminal. Naturally, the better you become at your trade, the nicer apartment you can afford and the more political power you have. The goal is to control the city, whether by campaigning for mayor or just having enough money to pay the right people to look the other way.
finally, there would be a pretty expansive forest area for a paintball/outdoor lasertag course, which would provide for everyone's FPS mini-game fix, which would of course be online compatable. hell, the initial vision was to have the whole game online, but i think thats a little more than a stretch.
so, that was our game we designed shortly after spending all night playing a bootleg copy of GTA 2 on our cpu. maybe too much violence for a nintendo game, but i'll be damned if that wouldn't be better than any GTA out there now (although looking at it now, it seeems like the only difference between our own game and rockstar's is that we're not all about mindless murder; its more career-focused. you see greater results by planning out everything before hand and getting the perfect shot for the front page story. oh and one last detail kinda ripped from pokemon: everyone has a rival , an eddie brock who thinks you're up to something and wants to find out how you just happen to be at every major newstory right when it happens. If you're not careful, he might catch you as you're changing back into your civilian clothes after a big heist, or he could even stalk your movements if you aren't mindful of your surroundings. taking back alleyways and complex routes could get him off your back, but a pivotal point in the game is when he snaps a super-incriminating shot of you and you have to decide whether to play into his wishes or call up a friend to have your rival erm... disposed of. (be a man, do it yourself; just don't leave much evidence of foul play)
sorry this is way OT but you wanted a GTA killer