What about an "AP Thronmail" item?

Thornmail gives +100 Armor, and reflects 30% of basic attack damage taken back as magic damage. So what if there was something like...

Runic Plate, which gives +30 Armor, +30 MR, and reflects 30% of ability damage taken back as physical damage with a maximum damage cap that scales with level (? x playerlevel = cap)

EXPLANATION: There has long been an outcry in the community that there's only one MR+Armor item in the game, Guardian Angel and this would be a second one. Furthermore, Thornmail is useless against bursters, AP champions, and most anybody who doesn't rely heavily on AA's. One could also argue that there is far less counterplay vs a burst mage who hops in and nukes you and walks out than vs a melee or bruiser champion who has to get up close and personal. As someone who plays a lot of burst mages (Veigar, Orianna, Anivia, etc.) I can say that CD's are the main way in which bursters are gated, but with enough CDR that can change greatly. This would diversify builds, add another hybrid Armor+MR item, and provide more counterplay vs AP bursters all at the same time.

Now I recognize that burst mages are almost always given lower health pools, and so reflected damage would effectively be more deadly to them than others, but Thornmail primarily affects ADC's and they traditionally have lower health pools as well. I have taken that into account when coming up with this item, and so let me elaborate on the details.

Namely, the %Ability damage reflected has to be high enough to not be negligible to abilities that deal lower damage but with multiple hits, but at the same time the maximum reflected damage cap has to low enough so that squishy casters like Veigar don't 100-30 themselves with a single combo. So I was thinking something along the lines of this:

(15 x enemylevel = Total damage reflected) or, in numbers, (15 x 18 = 270 maximum damage reflected per ability hit) at level 18. This would require an ability to deal ~890 damage in a single hit in order to cap off the reflected damage. Likewise, in a three ability combo a burst mage would deal ~890 damage to himself MAXIMUM (only if each ability hits for ~890 damage). And 1080 damage to himself with a 4 ability combo, all hitting at over 890 damage each. As you can see, it would be pretty difficult to reach that cap.