On his arrival, Season 2's theme song suggests sometime in season, though we don't know when. It's also a mystery if he will debut before or after Queen Bee. Considering that the Turtle Miraculous is in use already by Master Fu, my guess is that he'll come after Queen Bee.
Speaking of Master Fu, it makes an interesting question wondering why out of all the Miraculouses available to him and Marinette, including ones that are being used at least temporarily by Rena Rouge and Queen Bee, Nino is given the Turtle Mirauclous. It could be that the Turtle Miraculous' specific power is needed, but it is still an odd situation because Master Fu doesn't seem ready to give up his Miraculous currently, despite not being able transform anymore due to old age. What situation would make Master Fu willing to hand off his Miraculous for a (most likely temporary) period of time? That's an answer that should be very worthwhile to find out, I hope. There could be consequences for even just one time that Master Fu goes without his Miraculous too, like Hawk Moth captures him and he doesn't have a way to defend himself immediately.
As for the episode Carapace would premiere in, I have a few ideas. The first one would be if Nino's younger relative, Chris, gets akumatized. The downside is this would be following the same beats as the previous new holders, like Alya's sisters in "Sapotis" and possibly Chloé's mother and/or father in "Style Queen" and/or "Malediktor". My other less-likely guess is if Master Fu got akumatized. If Master Fu didn't have his Miraculous with him at the time for some reason, Marinette could give it to Nino, who helps to defeat Master Fu's akumatized form. Honestly, though, I would love it to be someone who wasn't expected that Carapace comes out to face, someone who isn't related to him or related to the Turtle Miraculous.
The Turtle Miraculous' power is especially a mystery currently. Protection is a common theory of the Turtle Miraculous' power, helped by the shell-themed shield weapon that Carapace appears to use. I would also guess that it could have the power of wisdom. I hope his superpower is more than just creating a protective shield, so I would love for his power to be wisdom or something related to turtle symbolism that isn't protection, though if it does end up being protection, it makes a lot of sense that I can't be that upset.
I suspect that something very bad is going to happen to Fu near the end of the season, forcing him to pass off the Turtle to somebody before it happens just to ensure it doesn't land in enemy hands. So far, I have no idea as to why he or Marinette would pick Nino outside of desperation, since he's done nothing as a character compared to Alya or Chloe.
Now that Hawk Moth knows he's around, it's highly likely that Fu will get kidnapped by him and/or Le Paon, but Fu will be anticipating this and making sure that the Miraculouses are either put in trustworthy hands or otherwise hidden someplace they'll never be found.
Maetch wrote: I suspect that something very bad is going to happen to Fu near the end of the season, forcing him to pass off the Turtle to somebody before it happens just to ensure it doesn't land in enemy hands. So far, I have no idea as to why he or Marinette would pick Nino outside of desperation, since he's done nothing as a character compared to Alya or Chloe.
Now that Hawk Moth knows he's around, it's highly likely that Fu will get kidnapped by him and/or Le Paon, but Fu will be anticipating this and making sure that the Miraculouses are either put in trustworthy hands or otherwise hidden someplace they'll never be found.
It's likely that Fu will have precautions in place to protect the Miraculous box if Hawkmoth finds out his exact identity (I can see him trying to create Akumas for that specific purpose rather than capturing LB's and CN's Miraculous, like one that can read minds), and if he couldn't fight, he'd send Wayzz away with his Miraculous to prevent HM getting his hands on it, and Wayzz would obviously go to Marinette since she's the only other person who knows about him that'd we know, and Marinette could also end up with the Miraculous box in her possession too. And if fighting against an Akuma get too hard, added to the stress of Fu being taken, Marinette could give the turtle Miraculous to Nino for help since she knows him, like she did for Alya.
Now that Hawk Moth knows he's around, it's highly likely that Fu will get kidnapped by him and/or Le Paon, but Fu will be anticipating this and making sure that the Miraculouses are either put in trustworthy hands or otherwise hidden someplace they'll never be found.
Nino being promoted to me seems very underdeveloped, which is a major problem if it doesn't get development for one episode or more before his superhero promotion. A lot of other characters in Marinette's class have had more moments to heroically help in situations with supervillains, like the girls riding with Alya in "Gigantitian" and the students (excluding Nino) saving Marinette from Befana in "Befana". Nino is a caring, morally strong guy, and it's not that I don't want him to be a superhero, but in-universe, it hasn't been justified enough why he should be purposely be given a Miraculous over other characters. Even if students end up getting temporary Miraculouses out of the Chinese zodiac batch, it is confusing why at least one of them like Kim, Juleka, or Max wouldn't be a better option than Nino based on their heroism in Season 2 alone.
Miraculous doesn't seem like the kill-off-character type, for better or worse, but Master Fu being kidnapped and/or akumatized would be dangerous, one or both making it so Hawk Moth can force out valuable information or capture some/all of the fourteen Miraculouses within the box. In a case where Fu is about to be taken away but has a split second, it would make sense to throw off his Miraculous to protect it from Hawk Moth's grasp, giving Marinette a Miraculous to temporarily hand off to Nino until they save Master Fu.
If Master Fu is kidnapped but any amount of Miraculouses are left behind, I think it's why he has been teaching Marinette about each Miraculous. My guess is that because of the circumstances, he is training Marinette as a new guardian in case something bad does happen to him. It's not the ideal system like the Order of the Guardians, but he isn't getting any younger, and Marinette has shown potential to be a wise, effective guardian so far with her choice of Alya in "Sapotis". If this is what is giving Marinette more importance in the overall plot, it works, though it can be argued whether or not there would be a better way to attach her to it more.
Alya began the season showing her more heroic qualities, it makes sense seeing how she became a hero first, the closer we get to seeing Carapase Nino will start showing his heroic side more. At least that is what I think.
Whenever Carapace appears, it's going to have to be sooner rather than later. There are only a few more episodes left in the season and Carapace has to appear before it ends.
Carapace obviously will not debut in Style Queen, and it would be very unlikely for him to debut in Queen Wasp since not only are the episodes Chloe-centered, but recruiting a new Miraculous holder to fight a rogue Miraculous holder would go against the whole "be careful who gets the miraculous" lesson. It's extremely unlikely Carapace will make his debut in either of the last two episodes, as those are the season finale and the big reveal of a "new" character. Adding Carapace at the last minute without any establishment would be cramming a lot in. So, of the four remaining episodes, here are my thoughts:
1. Malediktator: There is an extremely slight possibility (less than 1%) of Carapace, as the 2018 SDCC script read revealed that Ladybug would have to call on aid to beat Malediktator - However, narratively, it is a virtual certainty Ladybug will give Chloe/Queen Bee a second chance, and this time the changes in Chloe's character would start to stick. Since it has been said that this arc will focus on Chloe and her family, the possibility is horribly remote and next to impossible.
2. Frozer: There is also a minor, slightly higher possibility (maybe 2%) that Carapace will join in. However, since this episode is likely going to be a cross between Frightningale and Syren by combining both a celebrity guest star with the magic powerups, adding another Miraculous user would be too much for the episode to handle since both of the previous episodes had very simple plots even by Miraculous standards. Also, given that the production number is 217, it's unlikely that a new hero will show up when Rena Rouge is still a sporadic assist character and the default go-to girl.
3. Sandboy: This one is a lot more likely. A lot ot people are predicting that Sandboy will be Nino's akumatized relative, and just like Queen Bee and Rena Rouge, Carapace will come in the picture as a byproduct of a loved-one's villainy. Admitedly, a lot of this would fit just right...normally.
However, what makes this less likely to me is the actual plot synopsis of the episode itself. The plot synopsis suggests that all of the kwamis are going to be separated from their owners, which would render Wayzz unable to help someone turn into Carapace. Even if Wayzz stays behind, the whole point of the episode seems to be to strip Ladybug of her powers and force her to find a away to face the villain without any special powers or help. For her to get help would a little bit of a cheat.
Also working against the common theory is the production order, 223, which is often taken to be the canonical chronological order. Much like Simon Says and Volpina, for the episodes near the end of the season, there is a pattern for the stakes to be raised before the finale, and this episode will be exposition-heavy by its very nature. Putting another Miraculous user in it might be overkill, even though it would line in with the whole "particularly difficult missions" description for additional Miraculous users.
4. Anansi -- This is my pick. Unlike with all of the other episodes I've listed, we know Nino will appear in this episode, so that alone bumps it up to more likely than not. Unlike the shots and footage of Style Queen, the picture shown is still nowhere near finalized, which would indicate this episode is either far from ready or being kept under tight wraps. By contrast, we have more leaks and info about all of the surrounding episodes, which makes it odd why this one is being downlplayed. Note how Sapotis leaks were also tight-lipped about the Rena Rouge reveal beforehand.
The plot synopsis also reads, "Alya's big sister, Nora, gets akumatized into Anansi, a giant spider woman, and decides to show Alya how dangerous Paris can be."
If the villain is trying to show the danger of Paris, the inverse of this act is to protect, which just seems to be Carapace's theme. (Plus, if Anansi is trying to show Alya about Parisian danger arising from a twisted desire to protect her, that would make Carapace the perfect foil.) In fact, I can already see how the potential plots unfold. Perhaps, Ladybug does enlist Rena Rouge, but this time, things go horribly wrong (maybe because Anansi is a trickster like her nameseake and won't fall for Rena Rouge's Mirage). Or perhaps, Ladybug can't get to Alya in time and she needs another hero. Ladybug can't go to Chloe because of the Queen Wasp scenario that this episode likely takes place after it (and more importantly, before Malediktator). Nino is familiar with the situation, so he will want to help. Maybe Cat Noir is the one to pick Nino this time, since Ladybug might be having a crisis of confidence of how to pick an ally. This would lead to a moment where she'd have to trust Cat Noir even more than she normally does as he runs off and makes the choice.
What seals it for me is the episode production number, 221. This is after both Style Queen and Queen Wasp, in which we can presume that Chloe has either relinquished the miraculous or it got destroyed/rendered it inactive. There has to be a necessity for Nino to become Carapace, and not being able to rely on Rena Rouge (due to either Alya incapacitation) or Queen Bee (due to her antics) would be a straightforward necessity.
This is just my opinion, though.
We know Carapace will be in the Heroes' Day, so he'll have to debut quite soon then, and by the looks of it, he'll come before Queen Bee. I don't think it makes sense, but I've been ranting about the Style Queen delay all over the wiki. My guess would be Sandboy for his debut, seeing as it's most likely Chris, but Anansi would also be a good pick. In terms of its power, we already have powers of luck, creation, destruction, and illusion, so my bet would be the power of protection. It fits with the turtle. I don't know what his superpower would be, but I think that theme would be a good place to start with the Miraculous