Sakura's Rinnegan

Sakura and Itachi are the same age: Chapter 4.

Disclaimer: The only plot change so far is that Sakura is born in the same year as Itachi Uchiha. This is to make the age gaps not extreme when Sakura has relationships with older characters like Akatsuki members, etc.  

 3-4 years later:

Sakura Haruno at 4 years old:

Danzo sat in an underground meeting room in the ROOT Headquarters, with several men and one woman, and one Orochimaru.

"Consider Sakura as your own daughter, Fu Yamanaka and Torune Aburame, Tatsuma Aburame, and Yoji Aburame. I trust you each to babysit Sakura while I'm off doing official business, taking turns. Make sure the secret of her Rinnegan does not get out," Danzo instructed his henchmen.

"As for you, Nono Yakushi, you healed Sakura during your first day meeting Sakura. You know about her Rinnegan. You must also ensure that secret does not get out, or else I will personally slaughter your entire orphanage."

Nono nodded. "Yes, Danzo-sama."

"Good. Nono, you are in charge of healing my daughter from any side effects of what's about to get administered to her: Poison. Both Orochimaru's poisons, and the Aburame's variants of poisonous bugs; the poisons generated from each unique bug. Yamanaka, you are in charge of making sure that Sakura does not suffer any brain damage, from any of this. Right now."

"Yes sir," they all saluted, although the less-jaded Root members were internally cringing at the heartlessness of all this. Fu Yamanaka stepped forwards and placed his hand over Sakura's forehead.

"Shit!" Oni retreated and zapped herself out of Sakura's mind. Sakura was alone now (mentally.)

Fu Yamanaka began doing constant brain scans of Sakura's brain to make sure no brain damage happens and to cure anything that might happen. "You may begin."

Danzo pulled a cart of 100 needles and vials; this entire thing was a custom gift from Orochimaru. "Nono, stick each of these into Sakura. We will be here all night. We will be doing this everyday for a few months. We're trying to get Sakura immune to all poisons."

Nono stuck several of the needles and vials of poisons, all liquids mixed and invented by Orochimaru, into Sakura. Sakura felt the pain from each needle sting, and she felt Orochimaru's liquids, ninja viruses, and infections entering her body, but she did not react outwardly. Nono stuck more needles into Sakura, shuddering and healing Sakura whenever she had a fever flare up.

"Don't worry, Nono, Sakura. These are vaccines," Danzo smirked smugly.

"Danzo, her immune system is getting overloaded!" Nono protested.

"Oh, then we will do this over the course of a few months, until every single vial has entered her and until she's gained immunity to all poisions. One last thing, Aburames and Nono, meet up here again with Orochimaru next week. I'm getting Sakura immune to your bugs too. Make sure to steal all of the Aburame bug variants. Procure them somehow, whether through social engineering or outright stealing. Bring them all here. Then Orochimaru will create vaccines for them. Orochimaru will also bring Snake poisons. Orochimaru will be dorming here for a few months until we get Sakura's immune system to be as impenetrable as a God's."

"Aye, sir."

The following week, Orochimaru was there; Orochimaru brought along various snake vipers and venoms, along with vaccines and antibiotics. The procedure to make Sakura immune to all the poisons was a delicate task. Sakura was the first test subject for this program.

The Aburames had various bugs bite Sakura with their most lethal poisons, and Orochimaru and Nono Yakushi worked together and gave Sakura vaccines to the poisons, and healed all her injuries too.