Demonic Dragon: Harem System - Chapter 821: Moving towards Strax.

After dying, you wake up and realize you've been reincarnated as a background character who is judged as human trash.The decision to change your destiny comes instantly.Discovering that your identity isn't as simple as you thought, you feel favored by the heavens when you manage to become a powerful character.Katase: You won't fool me. I'll expose you for the pervert you are.Murayama: ...Thank you for saving us.Issei: Motohama, what happened to you? How are you so powerful?Rias: If you join my peerage, I wouldn't mind being yours.Sona: You beat me in the game. That means you're now my fiancé.Yasaka: Honey, Kuno wants a little brother.Kaori Houjou: Shinji-kun, let's go on an adventure.Kaede Kazama: Shinji-kun, thank you for saving me from that ball.Rie Sawada: Shinji-kun, I'll wait for you. You'll be my future husband, fufufufu...Changing known history and taking it in a direction not even his eyes could perceive, he didn't stop no matter what. Not even the other world was safe from his clutches.Lexia Von Arselia: My hero, and my future husband...Shinji advances without stopping.“I, Shinji Motohama, swear here and now that I will reach the top of both worlds.”

A scientific genius named Thomas Harrier was transmigrated to an imperial prince body after a car accident that took his life. He found out that he is in a world similar to our own with a timeline akin to early twentieth century. The body that he is occupying was the imperial prince of a failing empire who suddenly rose to power after a successful assassination attempt of his father. Unaware that the attack is just the beginning of what is to come. He faces many challenges throughout. Will Thomas Harrier be able to live his second life in peace while running an empire and getting it back to its former glory? Or will he succumb into a conspiracy lurking within the country's borders?

Expect some:

Cold war era techs.

Super battleship with advanced electronics, propulsion, and fire control systems?
Helicopters?
Missiles?
Modern medicines and equipment?
AC-130?
A nuke?
Challenger or Abrams or Leopard?
F-4 Phantom or MIG-21?
Colored TV's.

Everything!!!

PS. Cover not mine, I just found it on google.

Suddenly becomes a mafia boss in a superpowers world and you are only 3 years old?! Ainsley Sloan, a 20-year-old avid anime lover, suddenly transmigrated into a body of a toddler. The father eloped with a woman, and the mother was just a surrogate mom. The two left behind their 3-years-old baby girl to rule over a mid-sized mafia family!No, this can't be! Ainsley was forced to embrace the abandoned Sloan Family and bring their family to the top of the dangerous mafia world– else she died. But, worry not! With the power of ultimate luck, the help of her loyal subordinates, and the protection of her cats-becoming-human, Ainsley will kick any enemies coming her way! A late encounter with a certain system even paved a flowery path for her. With its help, Ainsley will be the best mafia boss, and an extraordinary one! When other mafia bosses smoke cigarettes, Ain has choco Pocky stuck in her mouth. When other bosses swirl their glass of red wine, Ain blows her cup of hot milk! When other bosses blow their gun after a fight...Ain changes her diaper. Watch the baby take over the mafia world with her unique baby charm! 7 years later. A certain shrewd mini boss: "Ain, how about becoming my fiancee?" The boy five years older than the toddler waved a limited edition BL pillow case as a bribe. A certain system: kiddo, accept his proposal! sucks his money! Wtf you two. Fiancee your a*s!! I'm only 10!

Lin Fan acquired the Sign-in System that led him on the path of the life of a god-level nouveau riche.