adaptive helm is the problem. it's overloaded with stats. it grants too much magic resist for the amount of reduction the passive already gives. you're effectively cutting out pretty much all of the damage from teemo, death fire touch, liandrys, cassiopeia, etc.
55 magic resist is just too much when combined with 20% damage reduction from DOT's. personally i think 30 magic resist fits the item more.
then there are all of the other stats on top of it to try and make it an alternative buy for abyssal scepter which is now abyssal mask. bottom line is both of those items cant have the same stats. it doesnt work. magic resist in general is heavily prevalent, but that doesnt change the fact that hp was reduced in exchange for raw defensive stats and where cooldown reduction was added to everything like 2 seasons ago the amount of burst potential went through the roof. katarina and lux also got their abilities amplified by passives that enhance cooldown efficiency.
so i agree that damage over time champions are suffering, but that's mostly caused by a larger change in approach towards the game and not so much because of tanks. burst was emphasized and DOT has literally no burst. it's part of balancing.
when compared to champions who can press every button at the same time get a kill then leave and have little to no negative effects it's a bit underwhelming to play a DOT champion.
there are very few items that completely negate an entire action or effect in the game, but adaptive helm is one of those items. at least in its current state. grievous wounds is another, but only the physical damage kind. true damage and % max hp damage completely counter HP builds.
if there's an item that countered burst (thunderlords, assassins, energy champions, etc.) then that would strengthen tanks even more. i think there needs to be a trade off at some point like how hydra has 2 forms and can only be built in one way or the other. perhaps the masks could be that same style and have their stats changed slightly to reflect their intents. in a way i sort of wish abyssal hadnt been changed, but instead a new item was added. reworking items like abyssal just creates more gaps in the item coverage.
maybe ghostblade and duskblade could be the same way as hydra also. then they could grant slightly more lethality in that single item.