Void staff needs to be nerfed.
It's nice that we actually have magic resist options for various passives but the reality is that going beyond 2 MR items gets you instadeleted by the enemies AD. You could argue that AD is too strong but this same scenario would be equally true if you didn't build MR or even built 1 MR item and having the AP delete you.
Nothing feels shittier than sitting on 180 MR and watching a Lux land a snare on me from FoW and taking 1200 HP off of me from her full combo. I realize that's absurd to say such a thing but investing that kind of gold into magic resist is supposed to prevent a burst oriented mage from doing that kind of damage to me in one combo in the first place.
AP reaps more benefits across the board than AD. It makes heals and shields stronger. It's a utility stat just as much as a damage stat but it feels like it gets balanced around the idea that it can only be useful as a damage stat when that simply isn't the case. I could accept a Ryze, Cass or Azir DPSing me down when they have void but I refuse to acknowledge a burst mage taking even 1/3rd of my HP in one combo when I'm sitting at almost 200MR - and let's be honest, calling Lux a burst mage is generous because she's more oriented on picks + combos and repeat so that damage feels even sillier considering that.

which still might be to effective at mitigating ap damage but also
and easily accessible armorpen both flat and %. Last whisper was changed from total Armor reduction to bonus a long time ago because AD champions simply didnt need an item that strong which destroys squishies and tanks alike and was the best choice in every situation but AP champions need just that.