Honestly. Damn ADCs and bruisers stealing our jobs! #BuildTheWall
Bruisers should build semi-tank and semi-damage, but that damage should be focused on dueling and lifestealing.
ADCs similarly should build all damage, but that damage should be focused around crit, attack speed and lifesteal.
Assassins should be able to oneshot if someone steps out of any ally range, but ADCs should not one-shot every squishy player during a teamfight. THEY should deal sustained damage. I'm not saying ADCs should be weak, I'm saying that Crit already works fine for Cait, Jhin, and Draven, and building Lethality is insane because with Crit, they didn't immediately burst a fed Zed.
I might just be biased, but I feel every role has been given their item type.
Tanks have armor, health, and in most cases just plain AD or AP.
Bruisers have lifesteal, duelist items, and health/armor.
Assassins have Lethality, AP, and some dueling items, depending on the champion.
Mages have AP/Mag Pen, mana, and some health items
ADCs/marksmen have crit, ASpeed, and lifesteal.
All of these roles have their optimum damage output with these items, and perform excellently with the items they should have, but when one role starts abusing a stat/item, then it makes the game unhealthier, and as a result, causes the item in question to be nerfed, and even further, starts to make the role the item was meant for weaker. Case in point, ADCs abusing Lethality. Everyone acts like assassins were the central point of nerfing Lethality, but everyone forgot Jhin, Quinn, MF, and maybe Cait? Not sure if she built it back then. The point is, ADCs and Marksmen played a huge role in the initial downfall of Lethality, and were the reason the ranged damage on Duskblade was reduced.
Now, how to fix it? Either remove Lethality and bring back the original Arm Pen, or make Lethality melee focused. Or even tweak the champions instead, then. Another thing is Riot always targets the stat, rather than the individuals becoming much stronger than intended with it.
But that's just my viewpoint.