The Basic Idea:Syrio is a Faceless Man. He never died after all. In fact, he might have disguised himself as the waif who's training Arya.
The Problem: While the two moments in the included GIF certainly look similar, they're actually pretty different. Syrio is flipping his weapons outward, and his palms end face up. The waif twirls hers inward, with her hands aimed downward. They hail from Braavos, so it could easily be chalked up to a similar fighting style.
Also, consider this: the fact that we don't witness Syrio's death isn't necessarily a sign that he's alive. Stannis Baratheon dies at the end of Brienne's sword in the season five finale. Though we never witness, she confirms his death in season six. It just goes to show you an offscreen death doesn't necessarily mean it didn't happen.