Ooh, I love Alternian worldbuilding headcanons! It's one of my favorite parts of being in this fandom :) I could go on and on about my personal HCs but I'll restrain myself to ones I haven't seen in this thread yet, right after this first one.

- My strongest headcanon is that the hemospectrum and all associated blood caste traits are the result of thousands of years of extreme eugenics, and isn't actually inherent to the troll species. If trolls got to exist without Doc Scratch and HIC interfering, and reproducing eugenics across generations through the whole "culling the weak and mutated" thing, the castes would be more evenly balanced in everything except for body temperature. (I like the body temperature thing :) not changing that one for anything) The lower castes' lifespans are that short because they're more likely to die young for various reasons, and trolls who live beyond a hundred sweeps probably have had their lifespans extended by some HIC life magic fuckery. Just like, ambient life magic. I'm sure you could swing that. Also, seadwellers can come in different blood colors naturally. Multi-colored mermaid trolls for the win! Smash the hemocastes!

- One biological (sort of) headcanon I have is that trolls have a very keen sense of smell, like ants. Grubs/wigglers instinctively adapt the pheromones of whatever interacts with them the most frequently or shows them the most care. This is how the whole "raised by animals" schtick even works on Alternia - abandoned infant trolls are capable of brood parasitism. It might also give a reason for trolls being inclined towards antagonism towards each other, and for adult trolls to be violent towards their young, which is, generally speaking, not extremely common. This is because, generally speaking, animals that don't provide any care for their offspring in some way are really fucking bad at perpetuating the species into the next generation. And personally, I base any headcanons I have about trolls on eusocial insects like bees and ants or even termites, all of which take care of their young until maturity. So, that's why trolls are apparently so aggressive towards one another - they pick up on different pheromones and identify them as "not from my colony/family/in-group, therefore bad". At least, that's assuming there's any biological justification for conflict between trolls and that it isn't entirely socially constructed, soooo...

- Another biological headcanon is that seadwellers and landdwellers are both sub-species of trolls who are extremely closely related, close enough to produce viable offspring. Hence, multiple blood colors among seadwellers and landdwellers. (This is not just an excuse to allow me to have a landdwelling violet and seadwelling burgundy OC couple shhhhhh...)

- A miscellaneous headcanon I have is that troll drugs are just normal human medicine. Here's the tumblr post that convinced me: https://www.tumblr.com/wispisntokay/721247173282512896/what-if-troll-drugs-was-just-like-normal-medicine

- Also, not all trolls can afford a recuperacoon. The troll equivalent of sleeping on the couch is filling up a bathtub with water and dissolving insta-sopor powder in there. Or you could just stick some sopor patches on your body; that's like sticking a pillow under your head on a long trip. It's uncomfortable and sucks and it's the worst sleep of your life, and no, you do not wake up feeling refreshed. I hate airline travel I mean what who said that

- Sugar is like drugs/alcohol to trolls. Have you ever seen those news stories of kangaroos who get jacked up on carrots because even that amount of naturally occurring sugar is too much for them? Yeahhhhh