The problem with Evenlyn is that she impacts the game simply by being in the game, without doing anything. She influences you to buy pinks because no other kinds of wards detect her. This problem is part of her kit and taking it away would be horrible for her. However her stealth mechanic could be changed.
I think that changing the way her stealth works at early levels (1-6) to not be undetectable to wards you BUY, but still invisible to trinket wards, until after 6 or even 11 and that would allow some nice buffs to her kit since she will have more counterplay.
My thinking is that her passive could be changed so she has different "levels" of stealthiness as she levels up eg.
1-5: only invisible to trinket wards, Fully visible to champions, green/pink wards and minions
6-10: invisible to normal vision wards, fully visible to champions and minions and pink wards
11-16: invisible to normal vision wards, 75% detection range for champs and minions
16-18: 75% detection range for pink wards, champs, and minions
This could allow for significant buffs to her early game as there will be significantly more counterplay to the champion because she isn't invisible to wards that you buy until later. This would also come with some significant sustain or damage buffs to compensate.
