Evelynn, the buff she needs
A disclaimer, I am not an assassin player or a jungler and these are just my thoughts from games I've been in with her, played her, and watched her being played.
From what I have seen her entire kit revolves around her W but the W is so hard to hit due to waiting 2.5 seconds then only having 2.5 seconds to proc it. I feel that all it needs is a much longer duration in which it can be proc'd by her. Instead of lasting 5 seconds (2.5 of which cannot proc MR pen or charm) it should last 10 seconds or longer. I might even suggest it lasts 10/12/14/16/18 but that might be too long.
I feel that this ability really defines her and the longer it lasts the longer it keep the opponent on edge. Picture an evelynn drive by where all she does is cast W but never reveals herself then goes back to jungling. How terrified is that target going to be for those 10 seconds? This could bring some really fun and interesting mind games to evelynn's kit at the same time making her kit more reliable as she would have a much larger window to proc her ability. True, her damage might be low but I think it is more important to put power into the thematic part of her kit first before buffing raw numbers and a more reliable MR pen proc with charm will like give her a much needed damage amp anyway.