Rebirth; I Choose to Love You for an Eternity, my dear. - Chapter 12 - ; Okay then...

Introduction to One Piece’s Bounty System: Tianlei crossed the One Piece World and become the nephew of the Navy Admiral ‘Sakazuki, ‘Akainu’, Tian Lei thought it was very outrageous until he found himself having a [bounty system] that can be exchanged for everything in the world of One Piece. As long as you kill or capture pirates, you can draw a Skills, weapons, and devil fruits. Such as ‘Navy Six-style’ ‘Overlord color Haki (Busoshoku Haki)’ ‘sense color Haki (Kenbunshoku Haki) ”armed color Haki’ ‘devil fruit’ ‘sword skills’ have everything!

- Description from Novelupdates

Just like how everyone adored heroes in their childhood, a certain young man adored those powers hidden in shadows. After hiding his strength and living the mediocre life of a mob character by day while undergoing frenzied training by night, he finally reincarnates into a different world and gains ultimate power. The young man who is only playing at being a power in the shadows, his misunderstanding subordinates, and a giant organization in the shadows that gets trampled…. This is the story of a young boy who had adored powers in shadows possibly eventually reigning over the world of shadows in another world.

Some say he is the bad apple in a basket, that he’s rotting everything around him with disgrace and corruption.

Some say he is the biggest scum of the South Zhanbu Continent, that he’s colluding with the dark sects to cheat, steal, and commit all manner of atrocities.

Some say he is the most wanted playboy. “Lock your daughters at home to keep Fang Xing away!”

To all of those people, “That’s right, I am that big rotten apple. Any problem with that?”

This brand of anti-hero is a young orphan with poor prospects and nothing more than the keepsakes of his uncles. He plots, schemes, and orchestrates all manner of villainy in his quest for power and revenge, unafraid to live precariously and make enemies out of those he really should not.

- Description from Novelupdates

The story is about a a hitman, Who dies because of his own recklessness and transmigrates into a game he used to play after his retirement as a hitman.I'm not gonna spoil any further, but i might answer few of your questions.Is MC overpowered?Nope, he'll become strong over time and all battle he'll fight will be because of his effort and techniques.Is there magic and adventure?Yes! It have both magic like normal isekai with adventure and fighting with monsters and many other things. There are other important things like “Essence Stone” But let's talk about it in the chapters.There will be harem?Not really, He will be in contact with many beautiful female and males but one waifu but i haven't decided yet.All i can say is, give it a try, I'll try my best to make it worth your time.