So some tank players, not saying all, complain about bruisers being the reason tanks are underwhelming and don't feel like tanks but in my opinion it's not bruisers that are doing this, it's mages. Right now there are 3 MR Tank items that exclusively grant MR and all of them are situational and can't be used to their full potential on all tanks which is why new MR tank items need to be introduced. To clarify I'm only going to go through the current MR tank items that exclusively give MR as their only defensive stat aside from health, so I won't bringing up Zz'Rot Portal or Gargoyle's Stoneplate since they grant armor and magic resist.
Let's start with Abyssal Mask. It's a good item, grants CDR, health, mana, some damage, and the Eternity passive which are all good in their own right but not every tank can really buy it. While not every tank being able to buy it and use it to its full potential could be fine if there were more MR tank items this is simply not the case. I'm not really a tank expert so correct me if I'm wrong but I don't see Poppy and Sion going crazy over an Abyssal Mask. If I understand correctly a couple of good tanks that use it are Ornn or Cho'Gath, champions that can benefit 100% off of things like the increased magic damage and the Eternity passive from Abyssal Mask but not all tanks can use it to its full potential.
Spirit Visage is another MR tank item. Once again it's a solid item, though Spectre's Cowl can be pretty underwhelming. But it suffers from the same issue that Abyssal Mask goes, if your tank champion doesn't have built in heals like a Maokai Passive or a Cho'Gath passive then it's unique passive does very little for you. This along with Morellonomicon existing makes it less and less useful.
Lastly Adaptive Helm, a good MR item once again but it suffers the same weakness as the others, it's too specific. This time it has very little at all to do with what champion builds it and whether that champion can use it 100% to its full potential but rather it comes to it only being good against AP champs with DOT effects. So while it's effective against maybe a Brand or a Malzahar, it's almost useless against a Leblanc. Again this would be fine if there were more MR options, having a MR item that counters DOT effects is good but not when the options are already so gosh darn limited.
All things considered a new MR item or two would be good for the game, it helps tanks tank better and have more universal options when it comes to building against AP champions.