Mr. Gu, Your Replacement Bride Is A Big Shot! - Chapter 1840 - : Probing

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.

He was once a great Immortal, but his attempt to ascend the Nine Heavens ended in failure. Reincarnated on the Blue Star, as he opened his eyes, he found himself with an adorable daughter. And a stunningly beautiful wife?“Daddy, can you cast a spell for me?”“I could even pluck the stars from the sky for you.”

Everyone rumored that Z country’s young master Jun was cold-blooded, decisive, and not close to the female gender, but he took in an illiterate little girl as the pearl in the palm of his hand.

However, the little girl brought her case to the table. An illusion, it was all an illusion. What not close to the female gender? Which animal was crushing her every night?

“Baby, let me love you.” The man undressed her with his lips. She retreated pitifully, “Don’t want, don’t want, you’re bad.”

“Women love bad guys.” After eating he turned around to wipe his lips. She kneaded her pitiful little waist, tears falling, “This one is not a woman, she’s just a little mermaid.”

- Description from Novelupdates

A swordsman hailed as the strongest has reached his end, and was reborn as an elven girl.

This is the story of the swordsman only knowing how to kill in his past life aiming to be the best healer as a girl.

In the final battle against a monstrous entity with his teammates, the strongest swordsman, the Hero’s mentor and the only one in their party incapable of magic yet the most skilled with the sword, sacrificed his life to end the battle once and for all. Reincarnated as an elf girl, the strongest swordsman decided to live a different life from his blood-stained past and pursue the magic that he has never used before.

As a girl, she apprenticed under the old potion maker and healer of the village to be like someone she had admired in her previous life while keeping her condition sharp with sword training as she had done in her past life.

After learning the ways of a healer and convincing the elf village that she is capable enough of leaving for the outside world, she hitched a ride with a party of adventurers passing by the village and went out to see how the world has changed.