I was thinking about the nature of the Slayers. Dragon Slayer Magic’s primary purpose is changing the body of the user into that of a Dragon, thus allowing them to use Dragon Magic. Doesn't all Slayer Magic work on this principle?
For instance, God Slayer Magic would change some of the user’s physiognomy to match a God’s, thus making the user a sort of demy-god, and allowing them to use God magic. To support this theory, you may have noticed that Zancrow, the Flame God Slayer, has red ringed eyes and sharp canines. These changes could be a result of the God Slayer Magic he uses. The ringed eyes could be the God Slayer's version of a Dragon Slayer's slit pupils.
The other two God Slayers, Chelia Blendy and Orga Nanagear don’t have these traits because their Magic is on a lower level than Zancrow’s. My guess is that once their Magic becomes stronger, these traits will appear to them as well. The same with Wendy Marvell, who currently lakes the characteristic slit-pupiled eyes of a Dragon Slayer.
Devil Slayer Magic should also work on this principle, turning the user into a Human/Demon hybrid. The first Ice Devil Slayer, Silver Fullbuster has sharp conines. We shall see how this Magic will affect Gray. Also I believe that Devil Slayer elements should have a different color than normal elements, like God Slayer elements are all black. Possibly blue or red.
What do you think?