The ONE easy to fix problem with Sylas
This;
W: KINGSLAYER Sylas lunges at a target with a powerful strike, damaging them and healing himself. If the enemy is at low health, the damage is increased; if Sylas is at low health, the heal is increased.
It's frankly over tuned as is, and needs to be nerfed next patch. He has 2 dashes and can steal the most influential parts of his enemies' kits. Now this doesn't have to be a balancing nightmare, and he weak early, absolutely. However this one ability is simply healing him for too much and it makes counter playing him impossible in a 1v1 situation when he has another health bar ready go and you're almost dead by the time it refills. If you're a burst assassin, he can heal between your CD's, if you're a sustain champ, he's sticking to you and hitting you back, baiting you into his heal, if you're a bruiser/tank, no chance.
In closing, this ability should be a clutch save mechanic, not an extra life. I understand that things such as Tryndameres and Kayle's ults are a thing, but consider that those abilities last a couple seconds, and can be played around in a 1v1 situation.
In the case of Sylas, he has double the mobility to stick to his targets, and taking down hp from 100 to 0 is not the same as baiting a cheese mechanic like Tryndamere/Kayle. Consider as well that Sylas may steal abilities like said champions given the chance, making him have an extra life beyond even what GA and Hourglass can provide.
Edit: My nightmare...
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ults

and more.