A suggestion for both a Legendary Skin(s) and the lore basis for it
The two skins:
Fae Guardian Lulu Fae Hunter Thresh
I like to think that the Shadow Isles were originally home to the Fae and were driven to darkness by the magical greed of summoners. We have evidence of this in the strange, magical fauna on Twisted Treeline: the altars are haunted by spirits of summoners and the place itself is a magical forest not unlike one perhaps inhabited by Fae. As the Fae, the symbols of the forest (i.e. life), have been driven away, what remains are spirits of death and darkness. Without a home, the Fae traveled to other forests across Valoran, one of which being near Bandle City. Pix happened to find a yordle of pure heart and imagination strong enough to do battle with the Denizens of the Shadow Isles and reclaim them for the Fae. Thus, he lured her into the glade to teach her fae magic and prepare her for the coming battle centuries from then She would have a very colorful, druidic theme or fairy theme to her, perhaps even transformed into a member of the Fae herself instead of remaining just a yordle. The color scheme would be mainly greens with purples thrown in (to contrast with her normal purple and red attire).
Of course, the other side of this coin is the Shadow Isles actors. I would love to give a legendary skin to Thresh as I prefer him to be the main actor for the Shadow Isles by virtue of stealing the souls of Lulu's parents. Thus, she would be motivated to seek revenge on him just like Lucian. However, the entire point of Pix secluding Lulu was to prepare her, and in his eyes she isn't ready yet. Well, screw that. She goes on anyway. I'd like to think that there is no true master of the Shadow Isles; they think for themselves. However, they send Thresh to do this specifically to lure Lulu in and therefore kill her before Pix can finish preparing her for reclaiming of the Fae Isles.
Unlike Lulu's skin (transforming her into something different), Thresh's skin would be just making him a more hardcore of what he already is like Bloodlord Vladimir. His color scheme would probably remain largely the same but make him be more incorporeal. As a Fae Hunter charged with slaying all Fae in his wake, his form would reflect the mysterious nature of his prey: instead of throwing a hook he is the hook. His hand is the actual hook. His body becomes less humanoid and more mechanical, almost like he has become the instrument of torture he wields.
The careful reader will probably have noted, "wait, this is pretty big. It's too big to involve just Thresh and Lulu. She's trying to reclaim the Shadow Isles for the Fae. Would that involve everyone else in the Shadow Isles? And what about her Yordle friends? Would they let her go by herself? Isn't this all more of an event instead of just two skins?"
Yup. [slayer-pantheon-rainbows]