I have a feeling it's going to be the Pointy-Eared Waver* instead of anything new. Adding it to the end of that trailer definitely feels like a teaser for more to come, and the timing wouldn't be hard to preview it one week and show more about it the next. It does look a lot like the next Mew-style legend. Plus, I have a feeling the term "phantom" isn't literal. Here, I feel they're just referring to something rare/hard to find/unknown/hard to see. I remember way back when, Special called Mew the "Phantom Pokemon." So I'm placing my bets on that guy.
*There should totally be an animal named this.