I'm currently working hard on this and I need some advice on which path to take. It would be fast pace action with hand to hand combat combined with shooting, maybe some bulllet time action with a concrete jungle urban sic fi theme. Either as a FPP or TPP, I'm still deciding.

Anyway the story would have a revenge plot based on the MC going after all sorts of criminals and bad guys after they killed his family. As the story would progress, the deeper you would get into the world of fighting different kind of bad guys, from gangsters to crime organizations, Italian, Russian, Triads and Yakuza to cults and world of assassins, cyber military and so on.

One option is the MC was a child a teen at the time like 13 or 15 and his entire family, parents and siblings murdered in his home by criminals including his dog that actually saved him when the bad guys thought they were all dead so the MC was the only one to make it out of the house alive. From there the story would pick up that he had gone under training for years of any means of training to become an efficient trained professional killer and you'd be going after and killing the bad guys one by one like Punisher style only more skill tactics.

Other option is he was an adult who was a former assassin/soldier but was hard for him to adjust to the civilian world and one day his wife and dog are killed by bad guys so this drives him to lose control and go after the bad guys same way like Punisher style.

Which story path is the better option?

Oh sorry about that. I should of posted it there instead. I was thinking of a few options for the mechanics. One is to have it where the MC was given some kind of drug that now has to keep getting more in order to survive which gives him an adrenaline rush, which I think fits with the emotional revenge theme. Another is that he had an implant in his system in his brain stem that would act like a chemical drug to suppress his emotional state from going rage which is hard to control now. Another is having some hallucination affect maybe his deceased wife telling him to get vengeance on those that did bad?  Another one would be that the dog that was murdered was his therapy android dog that would help him cope with his past but since the goons attacked and killed his android dog which helped him cope with civilian life and that he lived much now has nothing to kero him from losing it so it's taken over his rage or punisher like state to kill the bad guys. 

I'm not really sure how to approach a revenge mechanic so what kind of gameplay mechanic could be a driven playstyle that would allow the MC to fight bad guys out for revenge? Would some type of drug mechanic be interesting? Like some kind of brain implant that fights between both his violent and good nature conscience? Or something where he can exchange bodies or different prototypes?

What are some good possibilities I can look into for some type of mechanic that fits within the theme of revenge? 

You mean the brain implant one with two consciences struggling? Or the exchanging and possessing of other bodies? Sorry it might of looked like I wrote three choices but the drug would be a combination with the dueling conscience he's facing.

So either the implant conscience or the switching body to body using via mind transferring? Sorry if anything was misunderstood.

If you have a strong idea and connection to your neo-noir setting, try to let that inform your choices with both gameplay mechanics and the narrative. 

If the noir part is the important part, I would go with the brain implant route and make him an adult from the beginning maybe an ex-soldier or former criminal. As  classic noir generally is about criminals or soldiers or violent men somehow. Loss and revenge, all that stuff. Trying to change your nature. So the brain implant one and two consciences struggling is very noir.

If you have a stronger neo scifi element the changing body option seems better.

Or if you attached to a specific gameplay mechanic let that inform your decision.

But always pick the one you like the much, that makes the writing process easier I find.