%Bonus HP item? Bring on the unstoppable Leviathan
I'll start by saying there's a difference between a Soak item and a Tank item.
Soak items are purely selfish. They make the holder more durable but they don't make them more of a threat beyond the innate qualities of their kit. Thornmail, Banshee, and Warmog's are classic examples of Soak items.
Tank items, on the other hand, are items that provide defense AND increase the threat of the holder. Frozen Heart does this with CDR (MOAR CC!) and its IAS nerf aura. But Riot nerfed the Tank side of the item (weaker aura) and improved the Soak side (more armor). For shame, Riot. Sunfire is another Tank item. As is Abyssal Scepter.
If Riot really wanted to show some Tank love, they could swap out the Blasting Wand in Abyssal with a Giant's Belt to make it the MR counterpart to Sunfire cape. But that'll never happen because HNNNNNGH MAGES.
What we need right now is a STRONG Tank HP item to serve as a soft counter to %Penetration because there's no way to really itemize for optimum EHP at the moment. Against someone without %Pen you need 1500 HP per 100 resist to hit optimum effective HP values. That isn't too hard to pull off. Against someone WITH %Pen you need 2400 HP per 100 resist, which is nearly impossible on pretty much everyone aside from Cho, Rammus, and Leona. So with that in mind...
Leviathan
+
+
+combine cost
20% CDR, buncha HP, and
UNIQUE passive SPELLBLADE: On ability cast, your next basic attack grants a shield to yourself and the lowest HP nearby allied champion for 1.5 seconds equal to (percentage) of your Bonus HP. This ability has a 1.5 second cooldown.
UNIQUE active: For the next 4 seconds you take damage in place of the targeted allied champion (90 second cooldown).
So what do we have right here? What we have right here is a Trinity Force or Infinity Edge or Deathcap for Tanks. A Fighter isn't going to care about that active. A Fighter isn't going to be building enough Bonus HP to take full advantage of the passive. A Fighter is probably going to prefer grabbing Trinity Force (or even Frozen Fist) in place of this thing since the Spellblade passives don't stack. The shields that it grants helps to mitigate %HP damage a little without being too overwhelming. This is, without question, a late game fantasy fulfilling item for Tanks. Between the passive and active you are saving your softer team mates while being an awesome CC-dispensing juggernaut.
Wouldn't that be great?