According to the teaser, the champion will be able to shapeshift into different champions.
The teaser shows as well that the shapeshift may be often incomplete and can show what we can assume to be the champion's real appearance, this is especially visible with Ahri, as one of her tail is a reptile's. Other champions with shapeshift flaws have very floaty parts reminding of butterfly wings.
Now, looking at the Vastayan Tribes Tree, one of the tribe on top left have the definition of what is described above, "Reptilians Butterflies".
So my guess is that the "Colorful Mage Champion" is from the Strig Vastayan Tribe.
EDIT: ~~Looks pretty much confirmed judging from her appearance.~~
EDIT2: Nvm she's from a new tribe called the Oovi-Kat