If someone will ask, if Evelynn is in a good spot, they will be probably right. She IS in a good spot. She is probably playable and good and so on and doesn't need any changes. However, I think that she could have some changes in her abilities, so she'll be more personalized and so on. So, currently:
She is some kind of a demon or vampire, that pleasures herself with pain of surrounding victims. She stalks them and brings them to madness. As invisibility is involved in her kit, the rest doesn't reperesent much about her.
So, I've come with refreshed kit that will make her a merciless stalker:
Not only invisible, but she'll also have use of babysitting and trying to hunt the same target again and again and again. Her passive will be replaced, but the invisibility part won't be removed.
As some kind of a vampire/demon, she'll have some kind of healing from Ravage. Instead of making her a beast, I've made her this fiend she was supposed to be - and I've replaced attack speed, definition of fury/berserk/mindless onslaught with life drain, definition of all that's evil.
I've also have granted her an unique trait, that is infinite stacking, based on how much deaths her targets have.
So, instead of being just invisible, sexy lady, she becomes a merciless stalker, that will torture her target, make them cry and eventually ragequit. And this would be her niche.
Passive: Whenever an enemy champion dies, they get one stack of Nightstalk for the rest of the game. If an enemy has two Nightstalk marks, they become a victim and Evelynn cannot apply more stacks of Nightstalk this game.
If Evelynn kills her victim, she steals from them 1.5 - 2.5 (based on level) armor and magic resistance permanently.
When leaving camouflage, she gains bonus 75 movement speed for 3 seconds and her next attack within this time will also deal bonus magic damage. This bonus damage is increased by 2% for each death target has (4% if it was her victim).
Shadowstalk has no cast time and does not interrupt Evelynn's previous orders.
When Evelynn becomes detected until camouflage ends, she won't get the movement speed boost nor attack modifier buff.
COST: 55 / 60 / 65 / 70 / 75 Mana
Active:Evelynn slashes her target twice, dealing physical damage and applying on-hit effects with each slash. Additionally, part of damage taken by her target within 3 seconds is returned to her as health (healing doubled from victims.
This change was unnecessary, but I've improved it a bit, so she'll have good damage, thanks to all dangers she has to face with decreased base tankiness. The AD scaling was unnecessary though, but with removed attack speed buff, this will help her scale with AD generally.
This was necessary, as Evelynn without stealth would not be herself. So I've brought it here, so she can stalk her victims. Also, I've removed the massive speed buff (or rather remade it into a weaker one) with a damage buff on next hit (or Ravage).
Added healing, based on how much damage the target takes over time.
This was mainly because a bit of range increase to this ability would really help her. Not making it ranged, but being an assassin, she should not literally sin on the faces of her foes to use the ability. The mana increase is mainly to compensate for increased range.
Changed shield from 150 / 225 / 300 per champion to 100 / 200 / 300 (+ 25% AP per champion, doubled from victim.
And now it not only is more impactful, but also would look much cooler! Imagine lots of spectral, crimson blades slashing four times in a quick succession and the shred it causes among enemies. Also, granting silence versus victims will make then unable to run away from Evelynn, so she can take them down and grant them another two deaths, so later she'll be able to torment them longer.
So, tell me, what do you think about these changes. All feedback you'll write is good and will serve greater purpose!
PS: Thank you all for replies and your opinion. It's really helpful! (well, mostly).