During the S-class tests, Natsu ate a fire god slaying magic and was able to use a combination to be a Fire Dragon God slayer to know the guy out. So later he ate Laxus's lightning and now can call on the lightning to be a lightning fire dragon.
So, what I am wondering is could he not ever use the God slaying magic again? That'd make for a kick ass attack, something like breath of the Lightning Fire Dragon God.
Also, what kind of power could Wendy eat as a form of a power up like Gajeel just did with Rogues attack?