So, I know Roadmap is here and we are all hyped, but this small teaser was posted a few hours before.
First of all I though it would be a teaser to the Shonen skins.
Until I realized the symbol is NOT new.
This is Tianna Crownguard, leader of the Dauntless Vanguard before Garen (introduced in What Once Sailed Free).
And please look around her shoulder armor. This is the same symbol. This is most likely the family crest of the Crownguard.