Alright guys

So I was sitting today, staring at a wall, when it hit me
No, not the wall
>Dad joke
I suddenly wondered if Nintendo were going to, instead of releasing a game that had a NEW legendary in it, they released a prelude to X and Y
So, instead of having X and Y, then later Z is released - I believe this could be a possible scenario to shake things up a bit
They release a game that came BEFORE Pokemon X and Y. I don't know what it could be called, but that hardly matters. I know Pokemon have already introduced the new type to spice things up, so why not through a wacky order of game releasing in the pot as well?

This could also leave us with an evil organisation that hates us for something we did in the past, that we aren't aware of, and isn't made clear throughout the entire game.
Then, all is revealed as (perhaps this is a possibility) your rival slowly becomes distracted by the power that is given to him by these Pokemon, and begins to either force, or acquire trainers that begin to follow him. His force slowly grows into the evil organisation 'X-Ray' (I'm not creative with names) and they begin to research how to summon the terrible, demonic, and powerful Pokemon that is Dex Number 666
As you slowly take down his organisation one by one, they run off with hatred, vowing for revenge.
And in Pokemon X and Y, several years later, they get this revenge
Stronger Pokemon and more knowledge about Pokemon 666
This time, they summon the Demon Pokemon (perhaps a Rock/Flying gargoyle style monster?), but instead of capturing it as he hoped, your rival is nearly destroyed by the power of it. You jump in with your own Pokemon just in time, and the battle ensues
....
Food for thought