One Person's Loss...

We've all seen various shows, movies, anime, manga, or whatever where someone (usually the Protagonist) won't fully utilize, enjoy, take advantage of, use, or even try to enjoy something they have whether powers, a harem, or something else entirely.

Well, if they won't do so, then why not you?

Maybe you're someone who exists in their reality and as such you're going to take it all from them as they don't deserve or appreciate what they have.

Maybe you made a wish or made some comment something decided to make it a reality.
"HA, if I had those powers/women/capabilities, I wouldn't waste them like that idiot did" And it becomes a reality
Or maybe you were just put into that universe and since you know the characters, you know exactly how to get what you want from all of them (counting making the various women eat out of the palm of your hand).

Maybe you're a traveler and decide to usurp the role that a character has to fill: You're the hero now and as such the accolades, love, power, and opportunities are yours.

Or maybe you're just usurping their powers? Take the powers of the Avatar and Aang or Korra aren't important anymore and they become irrelevant beyond what you allow them to have.
Or maybe take Professor X's powers and slowly turn various women into your adoring slaves, lovers, sluts, and whore.
Or jump into a hentai and take the device or power they're using to control the various girls and now you can enjoy

Or maybe you just want their love interests/possibilities/options? Well a neglected/rejected person needs comfort and can easily be taken in and claimed.

Or maybe you're going just going to replace them in specific ways. Aang can stay the Avatar, but Katara's love and affection is only pointed at you with Aang never having a chance
Or maybe you're going to be an Airbender and you're actually the avatar and Aang never was, he's just a regular bender and You are the true Avatar that needs support to truly bring balance.
Or maybe Issei is still a reincarnated Devil and the dragon Emperor, but none of the ORC have any interest in him beyond a teammate or friend, all the girls are instead focusing on you or Issei is a reincarnated devil, but you're the Dragon Emperor and you're much more motivated and ready to fight enemies and tame women than he was
Or maybe you're gonna replace Riser, his peerage is now yours and you're engaged to Rias and as such you can get Rias to love you and never care about Issei and thus get an even larger harem.
Or maybe just replace Sirzechs as Grayfia's master, she's still part of a peerage, but she's your devoted maid and servant and maybe even wife.

Or maybe you're just gonna be in the same universe and slowly take away all that the person has since they're not appreciating it or using it effectively.
Maybe you're in Sekirei and you start working on taking away Minato's women because he's too much of a pussy to fuck them and give them what they really need, hell maybe you'll even woo Miya first and have free reign to take the girls of Izumo Inn without Minato ever finding out until it's way too late.

Maybe you're an enemy of the person and this is your way of getting : Strip away all that they have until nothing is left.
Perhaps you're someone who lost family to Sasuke when he was serving Orochimaru, and so you target not only him but Naruto as well for his part in keeping Sasuke from being tried for his crimes. Means it's time to turn Sakura, Sarada, Himawari, Hinata, Hanabi, Tsunade, and any other Uchiha, Uzumaki, and anyone else close to them into sluts while ruining the males.

There's a whole wide multiverse just waiting for you to play with and who says you only have to go to one universe?

(Public Story, feel free to add chapters as you please whether in first, second, or third person doesn't matter)