A simple suggestion for tanks.
If you stop and look, every single role in the game has an item or two that allow them to multiply, or at least just increase little by little over time, the damage that they're doing against everyone else... except tanks. I'm not saying that tanks need a way of increasing their damage, but rather a way of increasing how hard it is to deal damage to them. After a certain point, building more armor/MR isn't accomplishing much as the more you build, the more effective you make Last Whispers/Void Staffs against you. Plus, stacking armor/MR too much and not putting enough itemization into health actually hurts in the long run by reducing the effective health you're actually using.
ADCs have items like Last Whisper, or mechanics like stacking up crit, and using an Infinity Edge to further multiply the damage already being multipled by crit.
Bruisers have things like Black Cleaver, which lets them slowly chink away at your armor.
AD assassins have Youmuu's Ghostblade, which is just a large flat reduction while they go absolutely insane on you.
AP Assassins have Deathfire Grasp, which takes your hard built resistances to magic and reduces them to paper.
Mages have Void Staff, which serves the same purpose.
AP bruisers or tanks have Liandry's Torment, and the incredible synergy it has with Rylai's Crystal Scepter, which lets them effectively burn you away over time down to nothing while reducing your MR by a pretty large chunk.
Where are the mulipliers for being a tank?? Why is it that every other role in the game gets a way to make tanks ineffective, a way to make their offensive stats more potent without having to necessarily stack them through the roof, but a tank's only hope is stacking as many defensive stats as possible and pray?
So, a small suggestion, based partially from the excellent effect that Spirit of the Ancient Golem gave to tanky junglers near the end of Season 4. For those of you who don't know, Ancient Golem was given a unique passive that gave an additional 25% health on top of whatever bonus health you built from other items. Like a Rabadon's Deathcap, but for tanks. This allowed tanks to stack items that gave them armor and MR as high as they wanted without necessarily worrying about losing out on effective health, as each armor/MR + health item was giving the flat resistances while giving a large extra chunk of health. It eliminated the worry of when you were reaching the point that building more armor was overall a waste of gold that could be better spent elsewhere, and made the curve of effective health available to a tank a LOT smoother.
It also gave some interesting itemization options for champions like Zac or Volibear, who have kits that encourage a bit of damage output. For instance, Volibear is one of my favorite junglers. When the new Ancient Golem passive came out I was able to build that, build a Warmog's, and then run on the extra health given by Warmog's while I started working on a BotRK, building up a Warden's Mail if I needed some more armor before it was done, and then completing the BotRK and going straight tank afterwards. I was sacrificing a small amount of armor early on, compensating with that considerable increase in health, and because my tank items were, all in all, more effective I was able to safely run a single damage item that had a lot of synergy with Volibear's kit while still ending up about as tanky as I would have been if building flat tank before Golem got redone.
It was great.
But with the new jungle items, that wonderful effect has vanished! It was great while it lasted, but it ended far before it's time.
Riot, bring that back. Not necessarily that one passive as it applies to junglers, but a way of providing that sort of an option to all tanks. If every other role has a way of actively altering the stats that tanks have put hard earned assist gold towards building, give tanks a way of buffing their bulk to make up for it. Items that multiply overall armor or MR, or add a new stat for penetration/reduction resistance.
This would WILDLY increase the diversity of tank builds, and it would mean that more damage oriented roles would have to start making VERY meaningful choices in what items they want to build and when. A Diana who previously might have just been able to rush a DFG once the enemy tanks started building MR would now have to second guess that; she notices that the enemy Braum has built an item that cuts the amount of penetration that can be applied to him! Reduction then becomes a slightly better option, but Diana's current mainstay is penetration over reduction when dealing with tanks. Right now it takes no effort for people to itemize against tanks, while tanks are stuck just having to be on the receiving side of no end of penetrations and reductions.
Perhaps make it so that items like these don't give as many stats outright themselves, so if a tank builds one early to try and make himself unkillable going into the midgame, it's hindering his early game. If he builds it in the mid game trying to ready for the late game, he's now pushing himself off from the mass sort of resistances he needs to be building to be prepared for teamfights as the smaller skirmishes start breaking out. If they decide to wait until the late game, they're leaving themselves open to being penetrated and reduced to nothing during the mid game. Meaningful choices, for both those on the offense and those on the defense.
Right now the battle of penetrations and reductions against resistances is a tug of war, but tanks don't even have a way to hold onto the rope.
Let the tanks find ways to be tanks even when entire enemy teams are designed to make them fail at their one job.