Riot's Missed Subclass
Looking back over this list I believe that Riot may have missed a class on the borderline of Tanks and Fighters. If you look at the classes closest to the divide on either side, Juggernauts and Vanguards it's clear that some of the champions in these classes don't quite fit with the other champions in their group, or with the groups definition, but they do fit well with each other, which is where I think you have an emergent class.
The Vanguard class is a class that is literally defined by being durable and having strong engage. Defining a class in this way is not a great idea, because it's easy to point out additional example that should fit into the class, for example if Rek'sai is a Vanguard, why aren't J4 and Hecarim? I believe that there's a large split in this class based on what they do after they engage, some of them, such as Zac and Nautilus have more CC to stack on and continue to make a general nuance of themselves, while others like Sejuani and Rek'sai take a more aggressive approach and try to lay down strong sustained damage rather than just continuing to pump out more CC.
There's also one specific Juggernaut who utterly defies what a Juggernaut is, to quote Riot's definition with empathises
>Juggernauts are melee titans who relentlessly march down the opposition and devastate those foolish enough to get within their grasp. They are the only subclass who excel at both dealing and taking significant amounts of damage, but in turn they have a tough time closing in on targets due to their low range and extremely limited mobility.
Despite Shyvana being listed as a Juggernaut she utterly defies this definition. She is however very alike Sejuani, Amumu, Rek'sai ect. in playstyle.
I believe that these champs, as well as a couple of others fall into a class which falls somewhere between what defines a Tank and a Fighter. They're defined by their strong initial engage but bellow average CC follow up, strong bursts of heavily CD gated mobility, strong AOE damage in fights and fairly high ability to stick to champs in fights.
The champs who I'd consider to be in this class would be
Former Vanguards:
,
,
,
,
(hybrid Warden/Vanguard, Ramus is weird.)
Former Juggernauts:
,
(hybrid juggernaut)
Former Divers:
Formerly Unclassified:
Vanguard Primary/Hybrid:
Warden Primary/Hybrid:
Diver Primary/Hybrid:
Juggernaut Primary/Hybrid: 
I have a fairly large number of Hybrids because I'm overall more liberal about what I'd consider a hybrid than Riot, for example I'd call Ashe a Marksman/Disputer while Riot only has her tagged as a Marksman.