How about these?
Mind Parasites- The parasites used by Leo Akaba and "The Doctor" in Yu-gi-oh Arc-V to control the "bracelet girls," but mutated, with one being the master. The master gets into Ranma's head, and merges with his brain. Ranma remains himself for the most part, but with the desire to take control of his life, and use the drones to get a slave harem (along with being more open to doing sexual things).
The drones will enter girls' heads, and merge with their brains, but as they are drones, the complete merge will take time. A personality of a devoted love-slave for Ranma with the memories of her original self will be in charge, but her original mind will remain in the back of her head at first, unable to control her body or speak, while slowly becoming assimilated into the new mind until only the drone remains.
The master (Ranma) is able to take over and act through his drones' bodies. All drones can tell when someone else is a drone, or when Ranma's in control of (and thus treat the girl(s) as if they were Ranma, since any orders would come from him).
Wishing Sword- The sword used by Kuno in the manga, which grants three wishes to the owner. Ranma uses the first to remove his cursed form into a separate person unrelated to him, then uses the second to fix his life by changing the Tendo sisters, Shampoo, Ukyo, Kodachi, and Ranko into his openly loving, loyal, obedient harem. The sword only works for wishes from the owner's voice.
Imprinting Eggs- The eggs from the end of the manga. They make the ensnared see the first person they see as their master. They still have their minds and overall personalities, but are unable to even comprehend the thought of disobeying or betraying their master, let alone actually doing it.
Reality Journal- a journal, that when written in, changes reality to reflect what's written. Unless otherwise written, only the writer will remember the change if history is changed. The journal is indestructible.