How to fix assassins, juggernauts/bruisers and tanks
Change all tank items, nerf them to a third defensive stats (armor, MR and health and reduce certain passives/actives), and give a passive to tanks that increases said defensive stats to 3 times but certain passives and actives up to a two thirds, for juggernauts/bruisers just two thirds of the defensive stats and 3 times some passives and actives.
This excludes items such GA, Zhonya, Abyssal, BotRK, etc.
This way assassins can't build tanky anymore and go 1v5 and get that penta; bruisers/juggernauts can't kill tanks that easily and give some room to play them and don't deal too much damage as in capable of 100-0 squishies like assasins do, tanks don't deal that much damage but won't be melted easily either and remain just tanky. There, an easy fix.