Making other MR itemization more powerful - if the issue is lack of itemization responses to magic damage, rather than an issue that should be solved outside of the item system like the above two
Tank MR itemisation is abysmal. It has been for a long time. There's two good tank MR items, Spirit Visage and Guardian Angel, SV is only really a good item if you have healing however, but even then several tanks are forced to itemise for it because it's just the best stat stick available. The other 'options' are Zz'rot's and Banshee's Veil. Zz'rot's has so much of its power tied up in a gimmick that its use cases are limited for the majority of tanks while Banshee's Veil has a passive that does nothing to help tanks and is just outstated by SV.
To be clear here I'm not talking about support tanks, I'm talking about jungle and to a lesser extent solo lane tanks. Locket is not a usable item for these champs. It just gives too few stats, I can buy an SV and get 500 HP and 10%CDR for 30 Armour and 5 MR, in roles where I don't need to babysit and I actually have gold that's a worthwhile tradeoff.
I guess my main complaint is that tank MR items come down to being raw stat sticks unless you specifically need Zz'rot's. You buy what gives you the most stats. There's no useful passives or actives on large tank MR items. There's no Randuin's active or Warmog's passive which make you think about the game slightly differently. There's a shitty shield which does nothing because taking a single spell as a tank doesn't matter and a heal boost, which is worthless on 75% of tanks.
Locket used to fill a role as a tank MR item which had passable stats, a useful passive and active and made you question whether the stat stick of SV was the better option, that's gone now, I can guarantee you that in 95% of games as a tank now, SV is the correct MR choice, because the other options for large MR items are abysmal. Sure GA is good, but it's not enough MR by itself. The combo of MR and HP on SV make it far more efficient. GA tends to be best as a secondary MR item.