Basically, what pokemon do you think should have signature moves, but don't. For example, we saw this with Pinsir's Guillotine in Red and Blue.
I think that Hydreigon should have a signature move. Let's face it, he looks bad ass, and he's a beast, plus, he's the only hydra based pokemon, and being as hydra's are mythical creatures, they could have thought something insane up.