I think one of the first things Riot should do is overhaul tank itemization to make it as efficient as dps items are. The items just don't work well together due to passives stacking on top of one another. FH, Randuins, and Thornmail all have the Warden's Mail passive that is wasted if you stack them. And as you ramp up into late game stacking HP/Armor becomes very important as most of the sustained damage is coming from AD at that point in the game. Spirit Visage and Adaptive Helm do stack fairly well with their passives, especially when you consider how prevalent on hit stuff is now...but the games where tanks can afford to build double MR items are pretty rare.
DPS items are just far more well rounded too. So that when a tank is actually getting the items they need to be front liners in team fights, they are pretty much already getting outscaled by the mages and ADC's who are so good at shredding them. Back in the day the Frozen Heart into Spirit Visage rush worked really well because of the combo of MR, Armor, CDR on both items, plus strong passives, and mana from FH. But now true damage and penetration are so easy to come by in the early game that HP is sorely needed, not to mention the increased focus on damage which pretty much demands that tanks go Firecape or Iceborne to have any meaningful impact.
I know the concern with a lot of folks is a fear of returning to the days where tank tops and junglers were OP and ruled the meta. But the first thing to remember is that diminishing returns would still impact itemization and allow tanks to get outscaled by high damage in the late game, so no real change in that regard. If we just adjust the items a little bit so that their passives work cohesively together, and maybe spread out the mana/cdr a little better, the result would be that tanks are ready to fight right around the same time everyone else is, and it would take just a touch longer before tanks got outscaled. Additionally I think if tanks get their mid game power spike moved up a bit, I don't think you need to nerf Conquereor.
Here are ideas/recommendations: (again I think if you do this you don't need to nef conqueror which would maintain balanced for fighters/top lane)
1. Replace the Giant's Belt in Firecape with a Kindlegem for the CDR and up the combine price by 200 to maintain the same overall price
2. Release an armor item that has Catalyst as a component for the early trading and mana/hp pool. Effectively an armor version of Abyssal Mask. Not sure what the passive would be, but in terms of stats it should basically be the same.
3. Find a replacement for the attack speed slow on Randuins or Thornmail. There needs to still be an item with HP and Armor with the Warden's mail passive, so that it's not exclusive to FH, but the ability to stack both Randuins and FH, or Randuins and Thornmail without wasting stats would be sick for the 3 item spike of tanks.
4. Release an MR version of Righteous Glory
5. Make another pure HP item besides warmogs
6. Re-Instate Force of Nature and make an armor version
TLDR - A slight buff to the efficiency of stacking tank items would allow them to be strong enough in the mid game to not require the removal/nerfing of Conqueror, but also bring tanks back into the meta. It would emphasize their designed power curve of weak early and strong mid before getting outscaled.