Is Evelynn's perma-stealth even worth it anymore? Or should it simply not exist?
Evelynn gives up massive amounts of power for the ability to permanently sneak around and negate green wards from being used against her. Unfortunately, the payment she makes is far too high, and thus she is a bottom-tier Champion at this stage. S5 jungle is especially harsh on her because she can't clear more than 3 jungle camps before burning out and needing a recall, even with Ranger's Trailblazer.
Riot, perma-stealth might've been a novel idea at first, but it's clear that it simply cannot exist in League at this stage because it's too toxic. Either it completely snowballs, or it has no effect and turns the match into 4v5.
It'd probably be better if Evelynn got a new passive and Dark Frenzy instead let her stealth for short bouts like Kha'Zix's ultimate. There, now Evelynn can't map-sneak, but can be invisible in team fights when it matters.

being able to appear from literally anywhere is thematically awesome and I think can be worked with. The main problem with her right now is that there's not much about her you can outplay:
are useless against her, so her very existence forces you to either spend more money on
or play passively, and her main ways of dealing damage are autoattacks and her Q, neither of which are really avoidable once she gets in range. If she could get detected by green wards and had damage you could genuinely react to (aside from dashing out of range), then she'd be able to inflict a lot more damage overall and feel a lot more powerful.