The sleeping dogs system was amazing but realistically I think Rockstar would have a better shot at combining features the GTA series has already had with the elements from the WWE games.
In Vice City stories all you had to do was aim at someone from behind then press the button to grapple them and then snap their neck. So you could have that but when you aim at them you get different attacks with the combination of attack and different directional pad buttons
Taping the grapple button for a weak attack while holding the aim button on them or doing the same thing but hold the grapple button for stronger grapple attacks. These attacks all being able to be countered so you have to be careful.
You'd also have a certain amount of seconds (depending on how damaged the person you're fighting is) to drag them over to different objects like Walls, Cars, or Bar tables to name a few to slam heads into them or to toss the person into it such as the trunk of said vehicle or off the roof top of a building.
Holding someones head down under water to force them to drown to death.
Crashing heads through car windows
Throwing them out in front of a moving train.
Another thing. Ground moves. First of all the person you're fighting should not be as easy to kill as how GTA V made them out to be. In reality someone can survive a gun shot for quite a while and can be down but not completely done.
When they're down this is the time where you can finish them off with a Curb stomp.
Lay them out on the train tracks and then sit back and watch the show. (Train running said person over) Hell you could throw the person into the back of a car. Park the car on the train tracks and get an even better show out of it.
All types of possibilities