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Sinister Slush
already mentioned that they did give nerfs to tanks i.e Gragas, Sejuani, and Leona this patch. Leona and Gragas both having hit box reduction on their gap closers and Sejuani having a %hp based damage reduction on her W. I think that implementing some nerfs on Cassiopeia was warranted considering the fact that she is a pretty dangerous off-tank in the mid-lane position in the sense that she can take some harass from tanks in a team fight and still be able to zone the back line because of her AoE DoT damage and the health bonus she gets from items like Haunting Guise, Rylai's, and the mid-late game shield active bonus from Seraph's Embrace. Based on her strong performance at MSI I was surprised she wasn't nerfed in Patch 5.9 following the event:
I think that tank meta is a healthy meta change considering how much people were complaining that League of Legends was moving towards a "mobility creep" type of dilemma or a "League of gapclosers" type of problem where highly mobile champions with high burst and having instances of having no hit box were able to start snowballing very easily. Introducing more options to tanks like Righteous Glory was a positive change for tanks, however it still required tanks to work hard to coordinate with their teammates in order to maximize the benefits of the item or were solely used as a late game type of chase mechanic to finish a caught out player. Introducing Cinderhulk added incentive for people to play tanks since it rewarded tanks for doing their job properly and taking aggro on enemies who shouldn't be giving them aggro.
Counterplay has been developing around the "tank meta." The reworked Ashe is probably the best example as she lost a lot of her initial crit damage and immense slow appeal in exchange for offering more utility in team fights and objective control. Ashe works so well in the tank meta precisely because her kiting potential is far more effective against immobile champions without blink oriented gap closers since she can apply autos + W + R as needed in order to peel for herself. In team fights where tanks excel, she gains added benefit by being able to proc her Q on one array making her incredibly useful in punishing bad positioning. She is currently being spammed in low elo due to her skill cap being greatly reduced by her Q toggle essentially being tied into every auto being mana-less.
Similar champions that are able to kite tanks are Jinx who can lay down chompers and apply her W as needed in order to zone out tanks from the rest of their team. @
Sinister Slush
mentioned champions like Lucian and Kalista being strong for their ability to deal damage and having the ability to kite. I consider Kalista to benefit from her synergy with Ruunan's as she is able to apply AoE DPS when she is able to deal damage to tanks and their back line simultaneously. On the other end of the spectrum some people just pick Urgot as an ADC because he can build tanky (for an ADC), has pseudo-tankiness from his passive and W (ultimate), and provides utility enabling his team to take less damage-deal more damage-displace a badly positioned enemy.
I'm not sure how a tank building defensively can one-shot you honestly if they are only building defensive items. Or how a tank can both ignore cc and take no damage simultaneously. Essentially you're making Cinderhulk to be the equivalent of a tank oriented DFG when it offers far more counterplay than DFG offered on either assassins or magicians. In addition, tanks are most efficient when they operate in team fights which still makes them susceptible to aggressive split pushing, champions with good wave clear, and champions that have high mobility/escape capabilities. With the only strong counter consideration being Sion that comes to mind at the moment.
The problem isn't that tank meta is overpowered, but that team fight strategy is too linear in League of Legends as it comes down to a group mid and fight it out mentality which would benefit having someone like a Sejuani or Amumu with the Juggernaut Jungler item considerably. However, alternative compositions centered around obtaining quick objectives i.e. Sivir + Lulu + Yi/Udyr are still incredibly effective. The issue with tank meta isn't so much a lack of counterplay as people insinuate with tanky building, but the inability of the community to adopt new forms of thinking when it comes to teamfighting and strategy.