My choice tanks would be either Winston or Mannoroth. Both of whom are pure tanks in the games they’re from.
To me, even Reinhardt being a full-on is kinda debateable. One one hand, he has the biggest shield in OW made to protect his team and has a a large healthpool, but on the other hand, basically everything else he has is made to play agressive and deal damage.
His melee swings are some of the hardest hitting non-projectile attacks in the game, hitting for 70 per hit which is the same amount of damage as Mccree hitting body shots with his primary fire. He has his fire strike which deals 100 damage(I think) which is among the hardest hitting abilities in the game. He has his charge which is the single hardest hitting basic ability in the game hitting for a whopping 300 damage and is meant to 1-shot most of the cast. Them he has his Earthshatter which is a massive stun which opens enemies up to be taken down.
I mean, pretty much the only reason Reinhardt is even a tank in OW is basically because of his massive shield, without it, he’s totally a bruiser and could very easily end up being a bruiser in HoTs if translated.
Winston on the other hand is purely designed to be a tank, in OW. His incredibly low damage combined with loads of utility make him only able to be a tank and nothing else. Now sure, HoTs could easily give him the ability to spec as bruiser, but that doesn’t change the fact that Winston is purely made to be a tank. Additionally, Winston is easily the closest thing OW has to a main character, it’s actually quite a miracle that he hasn’t been translated to HoTs yet, especially with how much attention he gets in OW’s lore and how popular he is in the competitive scene of OW.
In short; I think Winston should be added to the HoTs roster before Reinhardt.
As for Mannoroth, the Pitlord is one of the few WC3 hero units who was designed to be a pure tank alongside Crypt Lord and Mountain King, and is the only one that has yet to join the HoTs roster. Now sure, the Pitlord did have quite aoe damaging abilities, but I think the ability that stands out most is Howl of Terror, which reduced the damage of enemies by a set amount. I think this would be a great design foundation for hero; a tank who mitigates damage by weakening their openents.
The closest thing HoTs has to that currently is through level 20 talents from Garrosh and Varian.(Demoralizing shout and Deadly Calm respectively) But I think that there is definitely room to design a hero around this mechanic, and I think Mannoroth should be that hero.
Mannoroth is a terrifying figure in the lore and his race is described to be a living war machines, known for their prowess and ruthlessness in combat. So in that sense I feel he should be similar to muradin or Arthas in that the capability for bruising is there, but first and foremost he is a tank.