I agree for the story side, I really don't understand why giving us "event legendaries"... The Delta Episode in ORAS perfectly matches what I'd like from these legendaries in-game.
That said, how can you say that Volcanion looks like a bad Pokémon? Its typing grants him 3 strong resistances, an unique trait, and other two resistances, while its ability let it absorbs HP from otherwise neutral Water type attacks. Steam Eruption is one of the best Water type moves in the game, combining the power of Hydro Pump with the utility of Scald, all of that backed up by more than decent accuracy. 80/120/90 is a solid bulk for an offensive 'mon, and 110/130 mixed attacks with a fairly nice movepool with strong STABs and good coverage makes it pretty good on mixed or special offense. Of course it isn't perfect, it lacks boosting moves, has 3 crippling weaknesses, it's vulnerable to all entry hazards and it's pretty slow, but those flaws are shared with a lot of other good Pokémons. All that said, we don't even know if it'll have a new form/Mega as Diancie and Hoopa (and Zygarde) did, or even some interesting tutor moves in Z... To me, even as it is, Volcanion looks absolutely promising even without being flawless.