Miscellaneous ideas.
You know my thoughts, but I'll post the ideas that we came up with for the Mistress Nine idea.
For a humorous route:
-Mistress Nine sees Happosai's route of gathering energy, and decides to imitate this. This can potentially lead to all kinds of amusing situations my favorite being Ranma finding all sorts of underwear on his person and in his personal items. Then (as Ranma, by this point, doesn't know that he has Mistress Nine in him) Ranma starts to get paranoid that he's unconsciously turning into a pervert like Happosai.
-Extending on the Pervert Nine idea, M9 stumbles upon an S&M club... and gets an idea. "Welcome to Club Nine, I'm the Mistress here..." ^_^
-Something else regarding Mistress Nine gather energy. Why not have her accidentally doing good trying to revive P90? For example, why not have Ryoga's lack of direction sense due to a curse... and have Mistress Nine drain it, curing Ryoga of his wanderings?
-Mistress Nine running into Kuno/Mousse/Ryoga, realizing that they could be good grunt workers and then enslaving them? It would be rather... amusing to have Kuno start calling Ranma "Master", or something to that regard with everyone wondering what the hell is going on.
-Another funny thought is Mistress Nine hiring... Kasumi to work at the club. And surprisingly (or maybe not so surprisingly) she's... good at it.
-Nabiki is also all right for the above option. But to add more humor, Akane isn't, and Mistress Nine points out that fact... to which Akane takes as a challenge. So, Ms. "I'm an S&M Mistress too!" decides to make Kuno her crash dummy. To which he starts to become frightened of her, and thinks it has something to do with Kodachi. And, quoting GH's spin on it:
"My dear wicked sister, how could you corrupt the fair Akane Tendo into turning against me?"
"Brother dear, I have no idea what you are talking about. Now please go get patched up. You're bleeding on the carpet."
-Now, as Kodachi is quite obviously into S&M, she eventually finds out about Club Nine... and an idea of becoming friends with one of the Mistress' there... Kasumi. ^^;; (I find the thought of those two being friends... amusing, to say the least.)
Another amusing Kuno scene from GH:
"Dear sister, have you any idea why the fair Akane has become a queen of pain?"
"Oh, that. The Mistress of Club Nine said she is not fit to be a queen, being a mere peasant, so she is most likely using you to prove her wrong. A pointless endeavor indeed..."
-Ranma being a closest masochist. (Hey, it would explain a few things, no?) This could lead to Mistress Nine getting Ranma on her side once they know about each other.
-Minako needs a job... and finds one at Club Nine. ^_^ Again, quoting GH:
"The mere idea of having one of those detestable wretches around sickens me, but I must admit, she IS good with chains..."
On the other side of the spectrum, we have the darkfic. The fic going in a seriously direction and Ranma probably being influenced by Mistress Nine. Not too many ideas on this one, but the thought of a Ranma with even a trickle of M9's sadistic tendencies is... scary.
-Hotaru, while not possessed by M9, is on the side of the Death Busters. That is, Ranma gets an inkling of how strong she is, and trains her up to work with them (If he actually starts working with M9). Hotaru, being so attention starved, would likely start following Ranma around like a puppy.
-Mistress Nine allying with/using Happosai. Alone it's scary, I think, but Mistress Nine might take the draining a step further and try leaving a trail of bodies with a convenient scapegoat.
It's taken a bit out of contest, but the meaning is still the same.