A new Magic Resist item that solves a lot of problems
Let's face it, MR values in league 2015 suck. But the reality is, having anything too high for values in magic resistance can completely nullify the burst of any burst mage. While I think that's a somewhat silly approach to have (sorry I think there's something wrong with a full tank losing 3/4th of their HP to the press of a button from one mage), there the fact that after a certain point, that's true. And in reality, 150mr + is usually going to help front liners survive initial burst from APs.
The real problem doesn't result from burst mages, it's easy to itemize against them because it's a matter of having enough MR to survive their burst. It's more of a problem with mages/ap bruisers with low CD, spammable spells and after next patch, dot champions. We often refer to them as machine gun mages, dps mages, John Cena mages, etc. The reality is, think of extremely high dps mages like Ryze, Cass, Karthus and the next tier is along the lines of an Azir, Vlad, even Rumble. The fact is, 150MR vs these champions means absolutely nothing. Their spells are high in damage, virtually no cooldowns and have itemizarion that can also enhance their safety and durability. These are the champions that MR feels extremely lackluster against (and pro tip, next patch, it's only going to feel worse). Because you'd need in the neighborhood of 200-250 MR to even withstand just a few of their combos without totally being zoned out of the fight or killed just due to how fast they are outputting their damage. It's huge, it's low CD and it adds up and it adds up fast.
I've seen some people say things like AP Thornmail but that's a terrible idea, that would feel extremely shitty to play against. I've even seen people suggest a Frozen Heart style of MR item that increases cooldowns of champions in an aura (like frozen hearts slowing auto attacks/increasing auto attack animation CDs). That's also a terrible idea, that would feel even worse to play against. The only item at the moment that can prolong your fight against magic damage is Maw/hexdrinker with it's magic damage shield. In theory spirit visage should too but healing during battle is never reliable, not to mention Morellos exists and is common on all mana based mages.
I propose an item along these lines: over the course of time, the more magic damage you receive, your MR begins to increase by a percentage of the damage you've taken. Cap the gain at something along the lines of the value of 100MR (or just make them take reduced magic damage over the course of eating a bunch of low cooldown spammable magic damage spells). This helps with a few things and most importantly: it does not nullify burst mages as their magic damage is upfront usually in the course of 1 combo with a few spells. But this also helps you deal with late game powerhouses in magic damage dps and MR doesn't feel like complete shit and John Cena is lame and predictable. But this also allows you to still have a decent amount of armor so ADCs don't two shot you for investing in so much MR.
no, that passive isn't doing much). Health still works against AD too, so you're not selling out against the ADC either.