Sorry, but it needed nerfing it to the ground...
But some THP are totally warranted. You can easily get them by barkskin though.
Maybe moon druid should allow casting self transmutations in beast form. And give some bonus to keep concentrating.
Why did it need complete nerfing, though? To the ground? The concept of a character who tanks through having low mitigation but tons of hit points seems to me to be an interesting option. The problem is that from levels 2-4 the moon druid has basically too many HP, not to mention overly powerful offence to go with its extreme flexibility.
By 5th level up until 17th, the sub-class plays fine - most would consider it a strong option but not out of line with the best sub-class from most other classes. It still has lots of HP but the difference between it and other tank sub-classes has narrowed in that regard, while its low AC and static hit rolls become an increasing liability. By that point, the main value is in the sub-class's flexibility, as it should be. It's a capable off-tank if needed, it can shape shift to gain advantages in different situations, and if needed the druid can just function as a capable if unspectacular caster. It's an effective jack of all trades.
OneD&D is already moving the sub-class choice to level 3, so if nothing else changed, we would be talking about a level 3-4 problem. Do we want to gut the way the most popular druid sub-class is played for the sake of two levels? Why not try to figure out how to tone it down, instead?
So what would be reasonable, assuming that we still want moon druids to be able to tank ( I do; that's my bottom line and I think it is only fair to players who love the archetype - maybe they played a bear tank in WoW or something, whatever. So if you don't agree with the premise don't bother challenging me on it; we'll just agree to disagree right up front)?
The main issue is bear form (or similar beasts). Being able to adopt this form twice effectively gives the druid 81 HP at level 2. That's a lot! On top of that, it has two attacks, +6 to hit, for 1d8+4 and 2d6+4. That's also really good! Its AC is 11, which is terrible, so there's that - mitigation is terrible.
Against a typical orc (+5 to hit, 9 damage), that druid is going to last roughly 11.5 rounds of the orc just whaling on it. Holy moly, that is tough!
A level 2 barbarian tank has a variable AC depending on whether they opt for unarmored defence or not, but let's call it AC 14, 24 HP, and one attack for, say, 2d6+3, + 5 to hit. For additional mitigation they have rage, which is excellent and effectively gives them double HP against melee attacks.
Same orc, my probably crappy math has the barbarian lasting 9.5 rounds. Well suddenly the druid tanking is not quite as broken as we thought. And this is at level 2 where the druid tank has the biggest edge, mind.
Level 2 fighter, specced for tanking: AC 19 (chain, shield, defence spec), 19 HP plus second wind for additional mitigation, so call it just 25 effective HP. Same orc takes 8.5 rounds to kill them.
Level 2 paladin, AC 19, lay on hands plus healing spells are potential mitigation, though has to give up an action. Let's just count the lay on hands and call it 30 effective HP. Same orc is looking at roughly 10 rounds to bring him down.
So, just in terms of durability the druid bear wins, but it's not as broken as I thought. The problem is that it gets that PLUS strong damage, PLUS flexibility to infiltrate as a spider or take any of a huge number of other situationally advantageous forms (i.e. giant elk to rush most of the party to safety, etc.), PLUS spell casting when not in wild shape.
How do we make that more reasonable at levels 3-4 without totally breaking the class fantasy that a lot of players have developed over a decade? Can we come up with a generic wild shape that still lets them tank, but a little
less effectively than barbarian/fighter/paladin rather than more? And can we give them access to
some of the flexibility that allows so much fun, imaginative play without removing almost all of it?
Edit: the bark skin suggestion, assuming the druid is using the new version of bark skin, allows him to live just 3-4 rounds or so against that orc, with a good chance of being one shotted if it crits. It's gonna take a lot more than that to make the druid a viable, if slightly inferior tank!