First post, weeee. This is probably overdone but whatever.
So by this point everyone should know who Diancie is. But the question is, how does she fit?
The main rumor is that she's somehow related to Hoopa and Volcanion. My thought? No. Hoopa and Volcanion look like they're connected somehow, but Diancie seems to have nothing to do with them. It's some kind of Indian-African-Ancient-Civillization with a giant purple diamond. Not really similar.
Another rumor is that she's an evolution of Carbink. BUT she was confirmed to be legendary, and no legendary has ever evolved- and that makes sense. Why would they change it now? I'm guessing Nintendo just isn't very original when it comes to Rock/Fairy types.
But, it might not be a coincidence. What if Carbink is somehow related to her otherwise? It's not unheard of, unlike legendaries evolving. Phione, for example (assuming Phione isn't legendary- seriously guys MAKE UP YOUR MIND.) But still, not a very substantial theory.
The movie is gonna focus around her (and Xerneas and Yveltal, but probably mostly Yveltal and Diancie because of the "Cocoon" part) and Mega Evolutions, right? And her purple gems resemble the Anistar City Sundial, which is related to Mega Rings in some way, shape, or form. What if Diancie is that way, shape, and form? Maybe when you bring your Diancie to the sundial, she activates something. Maybe it reveals more Mega Evolutions that are unheard of (like Latios and Latias)! Or what if it triggers another form of Mega Evolution? The possibilities are endless! (Well, to a certain extent.)
I dunno. Those are my opinions. Tell me yours! :D