What kind of levels do you plan for the actual game? I think an open-world Metroid style would fit nicely.
If you don't have any story yet, here's my idea:
You and your girlfriend are happy, your life, your (and her) friends, everything seems to be fine. One Sunday, like every other month before, you meet them at a big mansion, you all are having a wonderful meal together, a mafia-like family. Then, suddenly, your girlfriend tells you what a pathetic loser you are, that she is cheating on you for months and years and all of your other "friends" start to laugh and joke about you that you didn't notice it, but everyone else did. They can't stand you anymore, too, so they start to punch and beat you up, drive you to a harbor, shoot you in the belly, kick you in the water and drive away.
You survive seriously injured and you begin your vendetta, your rampage. This is where the game kicks in, you start with only a knife and low health and your goal obviously is to find and kill all of them (they are the boss enemies). I am aware that the story does not really match your current theme.

EDIT: Btw. this roughly was the story of a prototype similar to your game that I once made, where you and the other guys in fact were part of the mob.