The problem with Evelynn
This is not meant to be an "Evelynn is OP" or "Evelynn is UP" thread. These are simply my thought on why Riot has been so hesitant to change Evelynn (buff or nerf). I never really thought much about Evelynn before I saw her played in urf (the one mode where she truly shines). She was one of those champions who was rarely played, because she can't carry as hard as other champs can. And unless a champ is broken, people tend to stop playing them. Like many assassins, Evelynn walks a fine line between broken and useless. And although Evelynn's burst is lacking (currently) compared to other assassins, she possesses one trait that the others could only dream for. Evelynn is able to stay stealthed for as long as she wants, as long as she keeps her distance from enemy champs and their vision wards. And unlike other stealth champs (Teemo), she can also move while stealthed. In a game where good communication and map awareness is key to success, this is difficult to achieve with Evelynn. Unlike other stealth champions, who can be seen before they enter stealth, Evelynn is only visible when she reveals herself (on her terms). With other stealth champions, you have a general idea of where they are before they enter stealth, so you can warn your teammates if you see them enter stealth/start to roam.
In conclusion, Evelynn is a champ with a lot of potential. Unfortunately, due to the nature of her stealth mechanics, I doubt Riot will ever give her any significant damage buffs. She is at best a utility assassin (one of a kind) used for her AOE slow.