My personal headcanons (pretty much every single one I can remember lmao, prepare for a long-ass post (GOD I hope I don't come across as an asshole for this):

- A troll's horns keep growing for as long as they live. I think this is implied in canon, because exceptionally old trolls like the Handmaid and the Condesce have absolutely massive horns.

- Very basic and common ik, but I think that grubsauce is made from culled wigglers. I used to NOT think this, but Xefros's "you don't ask questions about grubsauce" and the conversation between Dave and Karkat in meat about trolls eating babies but "only the defective ones" made me change my mind (despite neither Hiveswap or the epilogues being strictly "canon," I personally tend to take anything new about trolls from them as basically canon, while regarding the EVENTS of their stories as dubious at best).

- Limeblooded lusii still exist (the way I see it, they're wild animals, so there's no reason for them all to just stop existing the moment the limes did), and limes are still occasionally born but are almost always culled shortly after birth like mutants are, unless some miracle happens or a Jade decides to take pity on them. I do believe that Karkat wasn't the only secret non-hemospectrum troll out there.

- Amphibious limebloods (I am a FIRM believer that Fiamet is a secret lime)

- Trolls generally either file their claws down or let them grow out into points. They're much harder and thicker than human nails, and are made of chitin. Some trolls habitually bite theirs.

- Troll hair, teeth and horns are also made of chitin.

- The one piece of troll lore established in Friendsim that I refuse to acknowledge as canon is them having nipples. It makes no sense for a species that can't lactate to have nipples. I will die on this hill.

- When a wiggler pupates, the two rearmost sets of grub legs are reabsorbed into the body, forming grubscars. The ones in front become arms, and the humanoid legs are formed new, because they jutt out from the bottom of the body vertically instead of coming out of the side like grub legs (this is just my flimsy justification for why trolls have two sets of grubscars. A friend of mine suggests that, instead, they're from the three-segmented body of a grub being "mashed together" into a singular torso, and I'm sticking to my interpretation but putting this out there because I think it's a good and valid theory).

- Some troll clothing is made from wiggler silk. If a wiggler fails to hatch from its cocoon, it is turned into silk. No canon evidence for this, it just feels in-character if we're already using dead wigglers for something ruthlessly utilitarian.

- Troll horns serve as display structures. Generally, larger is considered more attractive. They also serve to indicate sex, but in an extremely subtle and esoteric "only they can tell" kind of way that no other species can really get.

- Transgender trolls are completely accepted by Alternian society and the empire. This is not due to any moral reason, but simply because it doesn't affect reproductive capability, so there's no reason for the empire to care. All HRT and surgeries are DIY, of course, they don't have universal healthcare. Trans trolls can feel dysphoric about belonging to a caste that is predominantly a gender they're not, as well as their horn shape.

- Trolls is bugs. A lot of my headcanons are based on the assumption that they're insect people.

- Female trolls are typically larger than males, because that's how bugs tend to work. In an inverse of how human gender roles work, they're typically considered stronger and more aggressive, but there actually isn't much of a difference when you average things out, and these expectations are largely social.

- Trolls keep growing for longer than humans do, and usually wind up being taller than humans, although much like humans, adult body size varies wildly, so it's not at all hard to find a human that's taller than a troll. Trolls can hit seven or eight feet without being considered extremely tall, but can also max out at five or lower.

- The glaring exception to this are purplebloods, who continue to slowly grow throughout their lives after hitting maturity without ever stopping, like crocodiles or lobsters. While the vast majority die in battle or to battle-related health conditions due to their role as the empire's enforcers, and thus a truly ancient purpleblood is exceedingly rare, really really old ones like The Grand Highblood can become the size of buildings. Whenever I picture him, he's always like 30 feet tall.

- The hemospectrum is a construct. While trolls, allowed to reproduce freely without any culling, would generally fall into one of the 12 hues on the hemospectrum, mutantbloods aren't actually super rare, and come in far more colours than candy red. Also, very rarely, sometimes trolls with completely standard blood colours belonging to land-dwelling castes can be born with the fins and/or gills of a seadweller. These are culled because, despite having a "normal" blood colour, their traits are considered abberant for their caste (ie, their existence would call into question the validity of seadweller supremacy and the supposed rigidity and biological merit of the hemospectrum), much like how non-ceruleans would be culled for obviously deformed eyes, or a non-goldblood would be culled for having two sets of horns. Like limes, mutants generally only survive if a jade takes pity on them, which is extremely illegal.

- "Runts" are a thing, and are also almost always culled. Karako was a runt.

- While trolls, like humans, are omnivorous, the "ideal" troll diet puts far more emphasis on meat than veg. Trolls can eat raw meat without consequence, and have stronger stomach acid than humans. They can digest blood and small bones, and are able to produce their own vitamin C like most non-human animals.

- Because they are nocturnal and used to dimmer light conditions, troll eyes are larger and more sensitive than those of humans, and can see better in the dark. If you put a troll on Earth during a clear sunny day, they would find it overwhelmingly bright and be squinting constantly, but they'd probably be fine if they put on sunglasses. Their skin would actually burn less easily than that of humans, because it's evolved some resistance to the harsh Alternian sun, which is a real hydrogen bomb vs coughing baby type of situation.

- Trolls can't tan, and all have basically the same skin and hair colour barring mutations like melanism, leucism, albinism, etc (which are culled anyway). However, troll hair CAN vary in thickness and texture, and natural blood-coloured streaks are an uncommon but not rare thing to have (which I believe because, in the scene where we see him as a wiggler, Eridan has his violet streak despite just being "born." I also believed this before finding that out, because it's fun).

- Drones bleed black (which I believe because His Honorable Tyranny seems to be some sort of drone, and he bleeds black), can see in infra-red, and sound like the werewires from Deltarune. Flight is not an ability they naturally have, it's added to them alongside various other cyborg implements once they reach maturity.

Wall of text over... *GASP*

I'm hoping this prompts discussion, please feel free to reply if you disagree with anything I've said or have any thoughts!!