When I say John Wick, I mean the movie character and not the game designer. Granted, some of your players are going to want to be game designer John Wick - and that's ok. Like on Monday when I said your players want to be John McClane, I don't necessarily mean they want to be the character specifically, but the do want to have some of the key elements of the story for their characters. That said, lets go into why your Players want to be John Wick in your game.
It's About Recognition
If Die Hard is a story about perseverance, than John Wick is a story about recognition and the past. Recognition isn't as central a theme to the entire plot of John Wick as Perseverance is to Die Hard, but it is still there. When John Wick's name gets mentioned, people pay attention. A Russian mob boss is suddenly ok with someone hitting his son because his son harmed John Wick and that is an offense so stupid that it makes it ok for someone to hit his son. When John Wick walks through a syndicate controlled establishment everyone has the same question, their voices layered with equal amounts of fear and hope: "you working again?"
People know who John Wick is. They know what he can do. They respect, fear, and love him for these things. And this is what your players want.
What Have They Done?
In your game what have your players done? It's not that long into many D&D games when the PCs take out their first kobold camp that's been killing miners, or de-evil wizard a tower thatis preventing trade. The nature of D&D means it is only a matter of time before your PCs kill a dragon that is terrorizing a countryside.
Other games have PCs living through impossible scenarios, racking up intense kill counts in fight after fight, and always coming out the other end alive and ready to take on the next threat.
Simply put, by the very nature of being PCs your PCs become noteworthy people. Are you treating them as such?
Execution is Hard
Keep in mind the execution on this is hard. You want to show recognition, but you also can't have villains appear as push overs who are cowed by the reputation of the players (at least, not all the time.) But there are other ways.
Have the PCs enter a tavern to hear someone telling a story of their deeds - modified of course. Have other adventurers thank them for handling a problem that maybe they couldn't. The fighter of another group mentions he'd been part of a group that tried the Evil Wizard's tower, and lost 3 friends to it, so he's glad it's been dealt with. Maybe the local baron sends for the PCs specifically because they've killed a dragon before, and surely this Adult Red Dragon will be no more difficult than the young White they already slew?
Show the PCs that the world recognizes their deeds, and respects them for accomplishing it. Have their past come back to bite them from the same source. They'll enjoy it. It will make the world work better. And if nothing else, you can use it to let the players get bonuses to intimidation checks which they'll also love.