I don't know man.
AP itemization is already pretty solid, and we're seeing mage tops like
, mage mid-lane picks like
, and mage support picks like
within the current meta.
Also - mages usually have kits where they aren't stuck with only 1 way to solve a problem. Vs. a tanky team, they can often siege and poke down tanks until they're essentially squishies, kite a tanky team until they're low enough to burst down, or utalize a solid amount of magical sustained damage to cause large health pools to only delay the inevitable.
It's not rare to find mages like
can do all 3.
Anyways - poke/kiting/sieging is generally always an option with mages, as mages are frequently the class with the most range.
Also - most "tanks" are actually fighters, and fighters are among the worst team-fight champions in the game, where come late game mages are often among the best.
In lane towers and minions causing issues vs. auto-attackers also tends to make most otherwise solid duelist fighters get beaten by someone with range during the early lanning phase as well, and come late-game when everyone starts grouping: 1v1 power isn't all that useful.
It's also worth noting magic damage already has a free
on physical damage, seeing as MR stays at 30 for all ranged champions through bases and average level 18 armor tends to get to 75 - squish or not. Lowest is ~60, and armor is often much easier to accumulate through itemization than MR.
And physical and magical damage often aren't good parallels to one another, so offering parallel items runs into issues there. Magic damage scales much more with levels and less with items than physical damage, where physical has a tendency to be pretty weak through bases and scale monstrously with items: making monsterous physical damage scaling items more justified and monstrous magic damage items less so.
Now - AP assassins vs. tanks and fighters have serious issues, as poking and kiting often aren't realistic options for them and assassins can fall into auto-win or auto-loose scenarios.
I'm not seeing mages in any dire need of better itemization to deal with defense.