I think one of the most important "themes" of Evelynn is how ambiguous she is. There's a lot of contrast, in her character. Even if her current model is in shambles, it's not hard to figure she's supposed to be alluring, not in a "gorgeous" kind of way, but rather in the "seductress" genre. Yes, she's breathtaking, and would appeal to a lot males (and females), playing on their fantasies, if only she wasn't always invisible. She's a beauty no one gets to see (except on battle, but I'll come back to that later).
The other thing that strikes me is how "precious" she appears: she obviously likes to take care of herself (I mean, the Heels), and, much like Cassiopeia and LeBlanc, uses her charms when dealing with other people. The contrast here, is how savage she is in battle: this precious lady is full of hate, feeds on the pain of her opponnents, doesn't use blades or axes or whatever to strike and ennemies, but rather uses her bare claws, and relentlessly pursues her target.
If her dominatrix theme were to go (and I would personally wouldn't be against it), I hope this contrast will stay. She's one of the most mysterious champions in the League, and there are many options available: is she, as someone else's said, a demon (Succubus?), an hybrid, or even a Daarkin? Was she a poor woman, left by her lover, who then went to the Shadow Isles to seek vengeance, only to become cursed? Maybe now she's relentlessly pursuing all unfaithful men, punishing them for what she sees as the ultimate crime (hence the "Widowmaker" title). Maybe the opposite, and she's just a kind of twisted creature from the Isles, looking to feed on all dark feelings in a "human" heart.
So many possibilites... I'm really looking forward to what you'll do with her :)