I'd just like to note that the apparent fact that Dragon Slayers can eat the magic of other Slayers to gain a permanent "alt mode" supports my belief that the Dragon Slayers are just dragon people. They're just glass dragons, children without any "breath", so they need to feast on the breath of a dragon to obtain it. Weather Igneel or Metalicanus are alive or not, I'm thinkin' they are the birth father's of Natsu and Gajeel.
Some of this is also partly comfirmed with Acnologia, who killed so many dragons that he became one himself.