The issue with MR items is that they're most effective vs. burst.
"Wtf how does that make sense?"
Let's look at armor items. It's essentially the same thing in reverse. Sure you have armor which counters both physical burst and DPS, but all the items are geared toward anti-DPS.
Thornmail - returns AA damage FH - slows AS
Only one armor item is geared toward anti-bursting, and that's GA.
With MR, you have a very different scenario.
Banshee's - blocks one spell. More effective vs. burst than constant DPS. Merc Treads - most burst mages need to lock you in place for their burst. DPS mages are still effected by this but less so because they're more capable of constantly reapplying slows like Azir Q, Orianna's R, hell even Cassiopeia with her R is still fine because of Rylai's reapplications. Hexdrinker - I don't need to explain this one. Spirit Visage is the closest you have and with Morello's not being used nearly as much anymore there's less incentive to try burst down sustainers low enough to apply Grevious Wounds. Plus, the sustain works equally well vs. burst and DPS.
If you've noticed, most of the midlaners pros love to play are Lux, LeBlanc, and DPS mids. DPS means TF (yes he's bursty. No he's not technically a burst mage, he just CC locks you to death like Orianna), Orianna, Azir, Lissandra, and 70% of Corki's damage output.
tl;dr if you wanna make MR items more effective, make one geared for fighting DPS mages, not bursters, who already have plenty of counter items.
Tf played for his map pressure and he is more burst than dps, Lissandra is clearly a burst mage who play for her cc, Ori played rarely but if picked she played because of her versatility, she is not a dps mage. Creating an anti-ap dps item right now would make a weak class even weaker. They should just finally tweak maw and ap champs will be fine
ult, or play someone else for now.
