Tank items should give less health and more overall resistances
This would give anti-tank options a more defined role in a team since they wouldn’t need to punch through 4000 hp. However, it would keep tanks tanky against non anti-tanks who don’t build or have armor pen already in their kit.
Examples:
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Increase Dead Man’s Plate’s armor to 80 from 60 and lower its health to 300 from 425 and adjust its gold cost and passive from there
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Increase Spirit Vissage’s magic resistance to 75 from 55 and lower its health to 350 from 450 and ajust its gold and passive from there.
Just something like this would fix alot of the fundamental issues that arise during tank metas with tanks just being unkillable due to their massive amount of armor and health. I also hope that they make their items more interesting and fun. It isn’t my hope that tanks become useless, my hope is that anti-tanks are given a defined purpose in league without completely killing tanks off.
items and 
are in EVERY game.
and 
reduce the effective HP they gives versus physical and magic damage respectively, compared to their live values when considered independently of the stats of any particular champion that might build them.
and 
for an example as they have kind of close health but are on the extreme ends of the spectrum for base armor.