I would hope that this is fake, a new element is not needed at all. We've seen the mane six grow so much through these two seasons, another element wouldn't fit at all. If they were to introduce a new pony and a new element then they would have to make it a convincing story to say the least. It would have to fit ALL there lore we've had so far and I bet that would be a hard twist to make, they would need some brilliant concept for a plot to pull this one off.
Overall, I wouldn't mind it if they were able to make it convincing, but I would.prefer if they didn't even try. It's really not needed.
They would basically have to explain why this new element wasn't there during all the trouble they've fought through so far, plus they would need a darn good excuse why Celestia didn't tell Twilight about this before.