After the Bloodmoon fiasco, one can only hope Riot is planning something really good for her next one. Some ideas off the top of my head:
1) **"Uncanny Evelynn"** (could use a better name, I know: potentially a legendary skin)
The basic premise of this skin, is that Evelynn is a robot that looks like a very attractive human woman. So, in her base form, she looks almost completely human, except for some seams and subtle signs of being artificial (think i-Robot, or perhaps more like Detroit: Become Human in vibe), with her tendril things being a scarf or something to make them look normal. Could have that kind of "off" tone to her voice that uncannily human looking robots have.
When she goes into her invisible form, however, you see _under_ her synthetic skin. You see her circuitry, wires, lights, etc. Basically a really gritty, wiry kind of thing (think Terminator, or how Scott Cawthon's animatronics look underneath). Malfunctioning/glitching voice over.
Or at least, that's what I envisioned, before realising this would be a problem... Making a wiry, gritty looking invisible form, would probably be impossible due to detail in game being difficult to implement/models being too small. So instead, it could more resemble Orianna, or Blitzcrank, or "Battlecast" in terms of how Riot have previously done robots. And for her more human looking form, taking inspiration from the Winter Wonder and Heartseeker skins that Orianna has. That would be more realistic, and doable I believe.
2) **"Heart-Burn Evelynn"** (Valentines Day skin)
A normal, Heartseeker or Sweetheart aesthetic for her normal form. BUT, when she goes invisible she catches fire, and her next ability used coming out of invisibility has bright, fiery particles. Simple, easy, fun, pun. Her charm symbol could also be the regular expected heart, but with flames near the top (kind of like this in terms of where the fire is on the heart symbol: https://www.vectorstock.com/royalty-free-vector/heart-on-fire-vector-722362). Just to cement the "Heart-Burn" theme.
3) **"Atlantean Evelynn/Siren Evelynn"**
It's pretty predictable, and I don't think I need to go into what this would look like. But I imagine Riot could make it work. Think of things like Atlantean Syndra, and Deep Sea Nami. The Atlantean look on her regular form would be pretty easy to achieve, followed by something that looks more like a Deep Sea creature of some sort for her invisible look. Again, fairly simple, I believe.
4) **"Lycanthrope Evelynn"**
You're probably starting to notice a theme with my ideas by now: I love the idea of her invisibility ult being used as a literal transformation. Again, we have a sort of humanish looking Evelynn, I can't really think of much else that would work. But when she goes invisible, she is cast under a "Full Moon" looking spotlight, and she becomes a seductive, wolfish kind of creature (ew, I know. But come on, it's would be pretty special).
What do you think? Would love to hear more great ideas for Evelynn skins. And I really hope Riot's next Evelynn skin makes use of her ult transformation more creatively to make something truly unique.