A Tank needs tools to take aggro , protect allies and TANK damage they don't need 12% Max HP damage like ORN
A Marksman needs to do sustained slowly ramped up damage, be fragile , have ways to siege
A Mage needs to do aoe or burst single target damage , control the battlefield way of path the enemy take ( Ziggs E), BE FRAGILE do some moderate cc
A Support needs to either PROTECT allies OR ENABLE kills for his team BUT NEVER DO DAMAGE
A Assassin needs tools to be fast get to his target and deal burst single target damage, NO CC, BE FRAGILE , NO INVULNERABILITY ( akali shroud )
A Juggernaut needs to be a raidboss always, BE SLOW , BE TANKY , dangerous if he gets onto you, KITEABLE
Problem with league right now is that this simple concept is out of balance, right now EVERYONE does crazy damage, and if everyone is dying so fast that means all you need is characters that do that damage even faster meaning slow tanks are unusable.