So I wanted to come back to this.
I'm not super-keen on the 2014 Druid but I have played them a number of times and I think it gets more things right than the 1D&D playtest Druid.
Re: your points
1. I think this is a good suggestion, but I also suspect WotC thinks it's too complicated. One of the major issues we've seen so far with the playtest packets is some fairly strange ideas from WotC on what is too complicated. Because they been both increasing complication (spell lists, all casters prep) and cutting it down in inconsistent ways.
But "Build-a-beast" is the right way to go if using a scaling statblock, yeah.
2. Disagree that no shapechanging fiction works that way. Some does - but that that does tends to be very high magic Wizard-type shapechanging. However, one thing that is extremely common in virtually all shapechanging fiction, but absent in D&D, is the "shapechanging heals you". I don't think ablative HP were the way to go, but say, Moon Druids at the very least should probably heal some amount on every shapeshift (it doesn't have to be huge.
3. Agree - this is the natural way to do it, and how most players envision shapeshifting into any animal when they hear about it.
4. They should be there from as soon as you can shapeshift. It's not the "tankiness" that's really the problem, though, if you're using your own HP, it's the damage. They should do 1 damage or 0 damage, whichever fits - or even be unable to make attack rolls. I don't think knocking people out of small wildshapes is good because it'll cause too many weird problems, esp. when going through narrow spaces, and it doesn't fit with the new "Your HP are your HP" design. Maybe make the Druid Vulnerable to physical damage when they're in a tiny form? I.e. double damage - I think that'd fit and give you a reason to shift out but also avoid a ton of rare-but-troubling issues.
5. Agree completely re: flying - on a form weak at combat it should be fairly low-level. Also it'd be lovely if WotC could just, y'know, for one second, realize how fast and far birds can fly lol. Like a pigeon, for god's sake, a pigeon, can fly at up to 90mph for hours on end.
Regarding #1 being "too complicated" I think if Wizards can give warlocks 30+ invocations in that class' description, why can't druids get maybe 20 special beast abilities to choose from? I browsed a list and there really aren't that many, not considering varying senses and movement types. Anyone planning on making a flying, venomous beast with flyby attack and pack tactics? That is the big challenge, in how to limit it. Maybe in addition to the list of abilities themselves, have some entries that showcase how to select them?
Maybe...
Land Predator form (canines, felines, and ursines, oh my, etc.): Pick two of the following and add them to the stat block. A, B, C, etc.
Land Prey form (deer, oxen, sheep, hippos), Pick two of the following and add them to the stat block. A, D, H, etc.
Flying Predator form (birds of prey, flying snakes),
Flying Prey form (sparrow, duck, goose, bat)
Aquatic Predator form (quippers, sharks)
etc.
I admit it could be messy. There is so much that would have to be written to make it all make sense narratively.
Regarding #2. How about: "As you activate your Wildshape to shift into a beast form, or during the duration of your wildshape, you may choose to channel healing energy into your body. You may only use this ability once per wildshape duration. If it is more balanced to have a small amount, the healing can be equal to one of your many modifiers (Wis, Con, PB, etc.). If a larger pool is ok, perhaps you can roll the dice equal to that granted by your Healing Blossoms ability. That could help people be familiar with their Healing Blossoms ability and remember that dice number by heart.
Regarding #4. Vulnerability is very intriguing mechanically, but I can see some saying that if a critter doesn't have vulnerability, why do I have it in critter form? What about, While in Tiny Critter form, if you take damage from an attack you are forced back into your true form?
Regarding #5. Out of nowhere... What if Aerial Form also has the ability "When you fall and you are not in Aerial Form, you may choose to wildshape into an Aerial Form as a Reaction after which you are considered flying instead of falling."
The more robust Wildshape becomes to cover all the wonderful shapechanging tropes, the more powerful the Druid gets on top of being a full caster. I doubt Wizards will change the full caster aspect of the druid as they are the flower child of primal casters and there is so much history of druids having access to high level primal spells. Would a warlock chassis work, where the druid had to choose between opting into more shapeshifting or casters abilities, where they could only get high level spells by speccing into them like Warlock's Mystical Arcanum?