Health: a good place to start for rebalancing the cast
We all are well aware that NT has made some questionable choices on balance. Buttercup having a 50% slow by default and Burnout with a suspiciously short CD, Mekko came in with a 50% damage reduction on Safety Zone, and Makutu's rework. Whether or not they properly tested these things before enacting them is not my complaint only that some pretty OP stuff tends to get through the dev team.
So health bars, whats wrong with them? Nidhoggr has more health than half the tanks, Zerocool has the most health of the supports yet is supposed to be the one who needs the most protection, Miko is a the brawler support but has the smallest health pool possible, Gizmo has the second most health of the damage category despite being reliant on teammates peeling for her, and the mods for increasing the health of certain heroes are questionable.
Nidhoggr is stacked. Damage (both melee and ranged (hopefully something done about his Shredder dps)), cc, lifesteal, and on top of all that he has a metric fuck ton of health. I play a tank build on Nid (increased health, increased lifesteal, increase burn duration) mostly because I think Shredder spam is boring. I will concede that playing him with 950 hp feels busted but Shredder is even more so. Bring him down to 800 at least and tone down Shredder spam (I have a whole forum post on that so I won't bother repeating myself here).
Zerocool has 800 health, 100 more than Mekko, he is one of two supports with a true escape as well as a wall, and he has the highest healing output. That is a fishy combo. In my experience the hero in MOBAs who has a "wall" ability tends to be really strong. Cuts teammates away from each other, cuts off escape route, and buys himself time by blocking the heroes who are chasing him. Then when you finally do get on top of him he has 800 hp to chew through. Sure, he has next to no self sustain, valid point as to why he should have so much health, but maybe that is what should change. Lower his hp to 700 and change Defrag. Make it activate sooner and heal much more or make it a constant effect. If you've ever let yourself heal from near death to full using only Defrag you are well aware of how weak it is.
Miko is the support who is expected to peel and protect her allies. Pretty hard to do with only 600 hp. If the person you are peeling off your Gizmo decides to turn on you instead then it doesn't take much damage to kill you. Kinetic Barrier is best for blocking Zerocool's Recharge beam since the barrier has so little health. If a ranged hero wants to kill you it doesn't take long for them to break the barrier. Miko also lacks a natural escape. Yes she has the mod that increases her move speed but that takes up a mod slot while Azrael's Portal, for example, does not. It's hard to help your homies when it is easy to over extend on a hero who naturally cannot escape unless you bring a certain mod or use Stalker. Increase her hp to 700 (yeah this would make every support have 700 health which is boring but that also feels like the best place for all of them) or revert her self sustain nerf. There is also an option of increasing her health pool while buffing Chain Heal, this way she still wouldn't get the full effect from self casting Chain Heal but she would get more health off self casting than she does currently not to mention she will heal her allies for more. She can't be this squishy without a reliable escape and be expected to function in her intended position.
Gizmo has 750 health making her health pool second to only Nidhoggr in the damage category. Maeve is close behind her at 700 but these two heroes are drastically different after that. Gizmo is an auto attacker that deals the majority of her damage through basic attacks, Maeve is a caster who relies on her Life Siphon. Gizmo has an escape on bouncer that she needs space to use reliably (even if that space is using an evade to dodge an attack and simulatenously going across a Bouncer), Maeve can escape virtually anything with Vanish. Gizmo has no sustain, Maeve does. Gizmo is the Zerocool of the damage category in that she can carry a team if she is given space so why the extra 50 health? Although this amount isn't game breaking like Nid's it is an odd choice. I'm not ending this paragraph with "nerf her hp" I'm just pointing out a weird health value. Gizmo is in a good place but I just have to wonder, why is she given this extra smidgen on health?
Health mods are really weird. The majority of them are 100, which is a fine and consistent number, but once you start looking at the obscure ones do they start to look off balance. Two come to mind. Miko's and Cass'. We already talked about Miko so to remind you she is supposed to be protecting her allies despite her tiny health pool but she only gets to increase her hp by 80 while the rest of the supports have more health and all of their health mods are 100? Why? Cass has the same role as Daemon, they both have the same base health, and they both have numerous methods to engage and escape. Cass can increase her hp by 60 while Daemon gets a nice fat 100. Daemon also deals substantially more damage. Granted, Daemon is much more "all-in" so in that sense him getting more hp is fine but why does the difference have to be 40? It would be nice if all of these health pool mods were a bit more consistent, not necessarily the exact same value across every hero but there is genuinely no reason to use some of these mods while other heroes rely on them. Miko needs her health pool mod but it is also the weakest in her role. Rough life.
Health in general could use some changes and I know that will cause some issues in balance. Obviously my interpretation of how it could be fixed isn't perfect and I am well aware that I could be wrong on a few of these things I brought up. Simply put, health and the mods related to it across the cast doesn't make sense and I believe this is a good place for the devs to assist weaker heroes and tone done that strongest of the bunch.