When Thunderspeaker takes human form, how do they do it? Are they possessing a Dahan, or have they created a body to inhabit? If they created a body, how much of a flesh and blood body is it?
The story mentions that there are some spirits who are too slow and powerful to interact with the invaders in a safe and meaningful way. Why isn't Serpent Slumbering Beneath the Island one of those?
Does anybody else have questions they want answered?
I think Thunderspeaker creates a body out of energy, iit's probably more like a ghost than a flesh and blood body. It is more the unique fact that Thunderspeaker has a recognizable human body instead a serpent of water or a bird of lightning, ect.
As for Serpent, we know that Spirits are shaped by what they do, Serpent was sleeping for millenia, probably leaking the power it had when it created the island. The Serpent is probably the most powerful spirit acting, and it's nature of sleep being stung to wakefulness is why it is capable of acting as it does. I doubt it even fully wakes up, which is why it's power is able to safely interact with the island instead of destroying it
And I'm sure I have many questions, but I can't even think of them yet
Thunderspeaker takes the form of a Dahan, it doesn't possess someone.
You got it!
Would it be possible for any Spirit to take Dahan form (if they wanted to), or is that something unique to Thunderspeaker?
My big lore questions are what the heck happened in the First and Second Reckonings?
I wonder why Vital Strength of the Earth takes humanoid form.
And how Thunderspeaker decides whether it wants boobs this time around when taking Dahan shape.
Lore-wise, it doesn't have to, but often does because it's a little easier for the Dahan to relate to.
( On a meta-level, because that's how the art decisions shook out. The amount of input I had on details of any given Spirit's appearance varied a fair bit from Spirit to Spirit, depending on vagaries of timing.)
It would not be possible for just any Spirit, no.
It's not unique to Thunderspeaker, though. Trickster Watches with Cunning Eyes certainly can, for one.
Actually, Eric, shouldn't that be "It would not be possible for just any Spirit as they are at the beginning of the game's timeframe…"
I mean, we've already established that the Spirits change during the course of the game, primarly based upon the powers that they take. I would think it might be thematically possible if, for example, during a game Lightning's Swift Strike takes a bunch of Dahan oriented powers (that aren't necessarily about scaring them away from a land, haha) that LSS might have 'evolved' into a being that has closer bonds with the Dahan and thus be able to take a more Dahan-like form.
Well, yes. That’s basically what happened to Thunderspeaker in its backstory.
Oh, absolutely. I can't answer questions about how your Spirit Island is after your game. Who knows what might have happened? 
(Though even so, I'd feel pretty confident in saying that there are a whole lot of much-smaller-than-the-playable Spirits who wouldn't be able to appear as a Dahan even after 20-30 years.)
I agree that Thunderspeaker makes its body, but i think its as flesh and blood as you and me. The spirits are made of nature, not pixie dust, after all. (I suppose the exeption is Bringer.)
Can you give any examples of some Spirits who can/can’t take human form at the begenning of the Game?
also, what about spirits becoming animals? or for that matter other element-y forms at game start? We know Sharp Fangs is a Spirit of Plant, after all.
No, Sharp Fangs is a spirit of beasts - Keeper and Green are the plant ones. And the spirits never become animals, they just take their form, as Sharp Fangs and Thunderspeaker do (note - humans are also animals :P). I expect any spirit could at least take the shape of whatever they wanted, but they don't have to - Lightning is just a mass of, well, lightning, and probably just chooses to shape said lightning into a bird-like configuration because of the connection with the sky. I suppose it's much harder for some spirits to change their shape than others, however. The spirits are kind of like sentient embodiments of the aspects that give them form (plants, rivers, dreams, etc), which is why some of them look like physical species of plants/animals. And some of them just look weird (but very cool), like Shadow :D.
Some of it is also probably the influence of the Dahan, they take the forms we see them as because that’s how the Dahan thought they should appear.
when I said “become” animals I meant “take animal form”. The reason I connected Sharp Fangs with plants is because lots of its Cards have them and it relys so much on Jungles.
P.S. Lure Of The Deep Wilderness also seems to be plant oriented.
I would quibble a little bit, that Sharp Fangs behind the Leaves is something of a spirit of plants - though, specifically, it is a spirit having to do with the interactions between beasts, plants, and humans/human emotion. I don't think it could just do things with plants on their own, but plants are an integral part of what it does and represents.
Keep in mind, also, that the elements have multiple meanings. Moon, for example, also repesents cycles, dreams, and transformations. The plant elements that Sharp Fangs uses may not refer to literal plants, but things like vibrance, growth, and interdependence.
Its name also states that it lurks behind the leaves...you know, hiding like a predator waiting to pounce ;).