Lifesteal is out of control

Find two champions that are similar, but one is AD-focused and the other is AP-focused.

They generally kill enemies at a similar rate; maybe the AD champ needs to get in an autoattack or two. But what's the main difference? The AD champ can heal while they are dishing out the pain.

The main benefits? AD champs heal a bit when they're in a teamfight, sure, but that's not the biggest culprit. What's really a problem is when a champion is fortunate enough to escape a fight with low HP. What does an AP champ do? They recall. What does the AD champ do? They hit one jungle camp with their Hydra and are 2/3 HP. Hit a couple more minions and they're full, ready to push again.

Enemy team committing to a fight and you can't make it? AP champ has to bust their butts to get there if they want to help. AD champ? Why not go solo dragon! With lifesteal and a single attack speed item (oh look, one item is all you need!) you can do it!

TLDR

Make lifesteal a Unique Passive.