Write who you know. So make your OC yourself. After all, who understands you more than you(...stop crying) and we all know you are an amazing person who is able to win the love and affection of everyone everywhere. But because you are very humble, refute all accusations that the OC is you.
Remember, Dark, gritty, bleak and moody are "mature" regardless of source material. Toss in Gore, swearing, whining, angst and misfortune into your story.[Sidenote: I want to see someone make a Neon Evangelion fanfic more bleak than the original]
What's an easy way to get "drama"? Rape! Toss one into your story with the subtlety of a boat crash to show how "edgy" you are. Have a character raped and overcome it in record time with only minimum angst(for your story of course) to show how "strong" he or she is. Or go whole hog(Oh God, don't bring animals into this...) and have your character rape a character from the show you are most attracted to. You know s/he would enjoy it and how day s/he ignore your advances. How dare they get creeped out by the weird shrine you have in your bedroom devoted to them and how everyone will rue the day they laughed at it....
Call anyone who complains about your handling of the theme for being a pussy and not "getting it". Throw in something about censorship while you're at it.
Crossovers. Even if they make no sense. Handle the crossover as you would with the source material. Have the different universes interact with each other in ways that in no way are consistent with the original characterisation. Don't forget to pair characters between universes.
In the original material, this guy raped an orphaned dog right after he ate his adopted parents with some fava beans and a nice cianti. He's just misunderstood! How dare the protagonist try to stop him from being free to commit acts of vileness. Obviously, right this wrong by turning the dog raper into a sad tortured soul and the "hero" into a no-fun lawful evil bastard with a grudge. Now he needs a love interest because "romance" and projecting. Howabout the hero's girl who you love because she is so sweet, kind and loving? But turn her into a cruel kitten kicking ***** "thanks" to his influence. Have this character rape her(see because he's a dog raper and she's a bitc- aw, fuck it, cut this joke).
And now, if you excuse me, I have to dip my fingers in acid for typing that and dipping my head in bleach for remembering all the bad fanfics to write this.
-Edit- Oh, one more. Why not? Wanna make an OC design? Take a pre-existing character. Change the palette. Most likely to red, black or pink because it's EDGY!