Well, that hint certainly was rather, but I was hoping for something a little bit more thought-provoking.
Now that I think of it, the skin I've always wanted since it was mentioned was adult Annie
.
Something like "Magus Annie". None of that "Annie who looks like an adult but still dresses like a five year old" crap. I'm talking one who floats with her fire magic and wears robe/dress and other magical effects such as jewellery. Maybe some long hair with burning tips for effect.
Each spell would become more ferocious and bright with rank, with a special visual effect when she procs a stun (I guess her Q would have to only be upon the spell landing). Her W would be a more intense colour, while Tibbers would appear in a burst of magical flames. In addition, the Tibbers would have an evolving model, looking worn and damaged at first and the damaged spots leaking magical fire as he levels up.
The skin would have a lot of special responses, including things ranking up her ultimate, Tibbers dying and stunning enemy champions with her spells. She would have taunts for enemy champions like Brand, Amumu, Lissandra or Syndra.
It may not outdo the last ultimate skin, but I'd love to see something like that.