Ryze's skin change is all one big tattoo that he draws power from.

I would say, in tristana's + all yordle's cases, it's just a racial thing, as well as warwick's (his was a divine curse, not a magical one). Evelynn... I'm not sure...we don't know enough of her lore. Soraka is definitely an ascended being.

Looking over the champions a lot of the really magical ones does have much in the way of ears to begin with, so it's hard to tell who has them and who doesn't. >.<

Gragas doesn't have them (oddly enough) while Annie looks like she's developing a set.

Different people are changed in different ways by magic, so its not always the ears.

So tempted to make a really suggestive comment about how it makes things bigger...

So basically extencive use of magic can result in your body mutating?

...huh.