Of course. We know MR itemization is kind of painful right now because you have to build a bunch of null-magic mantle items in order to get MR.
However, simultaneously, we do recognize that a single large MR item hurts AP Casters far more than any other class, simply because of the sheer efficiency of linear resistances versus linear damage scaling on AP.
Thus, the problem is two-fold: Make enough MR items such that every class feels good about needing to buy some off-items for MR.
Two: Make sure that the MR is wide-spread enough such that one rushed item doesn't hard-counter mages.
There's a bunch of sub-goals involved in figuring out which core items can go on what characters in order to fill their MR needs and retuning some of our current MR itemization to simultaneously be less abusive when stacking pure resists but actually a decent MR choice. (BV, for example, suffers from the spell shield cooldown being super long.)
EDIT: Downvotes why? Sion in general is in a bad state as he is.