I'm not sure the engage potential has increased in the "tank meta." We can look to IEM Katowice for this. Typically an international competition features a variety of regional metas meeting, and the winning team, it can be argued, plays the most optimal variation of the contemporary meta-style.
TSM won all of their matches at IEM with hard engage comps using a single tank or less. They ran Vi jungle, Lissandra mid, Maokai top, Sivir ADC, and Annie support. TSM's alternative picks (not necessarily at IEM) were Rek'Sai, J4, Lulu, Kennen, Sion, Zed, LeBlanc, Viktor, Jinx, Lucian, Morgana and Thresh. Some here have less engage potential/CC than others, but I do think the optimal representation of the pre-5.5 meta involved assassins and big engage to force favorable fights and delete the enemy team quickly. It was TSM's clearly most-favored style. Even though TSM didn't play it, Hecarim was also becoming possibly the top lane power pick at this time, who fits perfectly in with this mid game engage snowball style.
Fast forward to the tank meta, we still have high engage and CC, but we are seeing some teams trend more towards disengage with these powerful front lines. With lower damage and fewer assassins abusing the map against passive scaling champions, Karthus, Vlad, Orianna and Azir have returned. The late game meta shifting bursty ADC like Graves/Corki/Lucian out for autoattack ADC's like Kog'Maw and Jinx favor sustain supports like Nami. Urgot and Kallista are in. These make great ranged anti-dive or kiting team comps, especially with a Lulu or Trundle top or any frontline tank, as well as Nunu, Nidalee, or Grags jungle. There are plenty of kiting picks in the scene.
Disengage notwithstanding, power picks like Sejuani, Zac, Shyvana, Hecarim, Nautilus, support Kennen are all real. Even though Kennen's ult doesn't do much against beefier targets, as a support you don't miss out on damage picking him over a Morgana, but you have a much more brutal engage, burst, and lane trading. As expected, having more diversity in viable champions results in more diverse play styles.
Overall, is it really a tank meta? Lulu, Kennen, Nami, Janna, Morgana, Karthus, Vlad, Azir, Rumble, Leblanc, Zed, Nidalee, AP Kog'Maw are all viable picks. Bjergsen said on stream 2 weeks ago that if Viktor weren't disabled, everyone would be playing him. We even see Smite/TP Lee S...ok I can't go that far. At any rate, it just doesn't seem the picks are entirely one-sided in favor of tanks, perhaps the tank picks are barely surpassing 50%-60% of all viable picks.
Was this just a result of Cinderhulk? Cinderhulk heralded in tankier junglers, but actually the old jungle was dying anyways. Sure, Nidalee and Rek'Sai were good, but they're still good now. After J4 was bodyslammed with nerfs, the only jungle pick lost from the old meta was Vi, in exchange for a handful of tank junglers. Good deal for us, but the previous jungle meta had been progressively pushed to its most fragile state in years. Cinderhulk was the final Jenga block. At the same time, Cho'Gath was gaining popularity even before 5.5 as an assassin response, and Urgot received a major buff, both picks critically successful lane choices against the dominant Zed. Xiaoweixiao broke out Karthus before 5.5. Lanes everywhere were already burgeoning with ways to slow down the lane phase.
Back to the meta. I think we currently find ourselves in a fluctuating meta where mid game snowball comps are struggling to maintain the traction they once had against immutable beefy opponents. This leads to a meta whose definition encompasses more than tanks; it includes a variety of late game picks. It's a scaling meta. Just an appeal to semantics? Probably, but labels carry a lot of implications in their superficial interpretation.
While a mid-game win condition comp has relatively few potential picks (Hecarim perma ban) and is fairly unreliable at bullying more passive lanes to provide a consistently successful early snowball strategy, we still see some of them having success. Double AD comps can be run. You can with some effort scrounge together an early game comp roster -- albeit at your own risk. Still, most teams seem to be finding a way to incorporate at least one or more scaling picks into their rosters, whether or not they are tanky.
One interesting existing powerful scaling strategy that nevertheless has early snowball potential is the double smite comp, which especially in conjunction with a lane swap, can allow a team to secure a sub-5 minute dragon and start stacking them with incredible efficiency and a limited ability to contest from the enemy team. In standard lanes you end up with a massively farmed top laner, whose Smite is higher level than the other jungler, to bully dragons for your team. By 25-30 minutes, you can be threatening 5th dragon, a relatively early win condition.
How long will this diversity last? Who knows. With this many picks and potential comps vying against each other, as well as the inevitable innovations in pocket picks that erupt in an unstable meta landscape, it could take several weeks before players discover the most optimal play style, the most viable 2-3 picks per lane. Or maybe, just maybe, Riot has finally struck gold, the gold mine of diversity, where a variety of play styles each supported by a mutually exclusive cast of champions can coexist without collapsing into a single default choice.
Boy do I feel bad for whichever Rioter will be blamed for the patch that knocks us out of this sublime state.