Two years ago I had on plans a series similar to a XY walkthrough except for a huge different kind of plot. However that idea got scrapped, that included this character.

This shiny Sylveon, Sylvester, was supposed to be one of the main antagonist.

So if that pilot series was scrapped, and happened to him?

I transferring him to X+Y. However, just like the Zoroark, he won't get a real major role as the series is focusing the legendaries, you know. And by this timeline, he's still an eevee.
But that's only for X+Y! Once it will end, it will get a sequel. And once that sequel will be up.....Sylvester will be the main antagonist as first planned.