Let me use my highly scientific deductive skills to determine, based on previous X-plots, what X9 would be like.
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Alright. I got it.
First, a mysterious guy in a cloak. It's obviously Sigma, but we play along and pretend we don't know who it is. He does some bad stuff. Cue Maverick Hunters.
Hunters X, Zero, and Axl arrive on the scene. They look around and fight some giant mechaniloid, and head back to base.
EXACTLY 8 MAVERICKS APPEAR IN 8 SPECIFIC LOCATIONS SYMBOLIC OF THEIR POWERS. In the short time that X and crew were out wasting a wimpy giant, Maverick-Hunter HQ knows everything and anything about these mavericks EXCEPT that Sigma built them. EVERYTHING EXCEPT THAT.
X and crew proceed to lay waste to the villainous curs.
Halfway through, (exactly 4 bosses defeated), a crisis. Someone attacks the base. A shady new character who will be included as a new Biometal in the new ZXObligatorySequal (Name pending). X and crew kick his (or her? Naw.) sorry ass to the curb and get back to business.
X and crew pillage some more Maverick hideouts.
All of a sudden, they get to base and X goes "Hey! I bet Sigma's behind all this!". Almost on cue, Sigma appears on a screen and announces: Yes. He did it. Furthermore, you killed his poker-buddies so now he has to blow up the earth.
SIGMA STAGES. DA-DA-DA-DAAAAH.
We like Earth. All our porn is there. X and crew now attack Sigma's fortress, and GUESS WHAT. They have to fight the same 8 guys all over again. Oh, and that mysterious guy from before, who turns out to be under Sigma's control, but a couple charge-shots and Saber-wounds to the head will fix that right up.
Now we fight Sigma.
And now we fight Sigma again.
AND WE WIN.
...
Naw, I'm kidding. We fight Sigma some more.
And now we discover, it wasn't really Sigma!
So now we fight the REAL mastermind, because three Sigmas just isn't enough.
And now that we've beaten him, we get to fight him a second time!
And a thi-no, wait. Yeah. We're done.
HAPPY END.