There's not a lot information to speculate on.
First of all, which curses do we know of that are still not in the hands of players?
I can mainly think of 2 very important ones + 1, but technically there are two more things that can be considered a curse. For a total of 5.
The main two curses which players tend to ask for are: the skeleton curse and the "ghost curse".
Well, this is pretty self explainatory, we encountered a lot of skeletons and skeleton Lords in the game, also Ashen Lords which are very powerful Skeleton Lords who can some kind of fire magic.
To my understandings (books and lore videos) this curse is acquired by drinking something from a chalice; I have no idea what liquid this is, if the chalice matters, maybe it has to posses some specific magical properties, and if the place where you perform the ritual matters.
This is for me the most likely curse we'll get next as players have been asking for this for ages. But, I've been thinking this would've been the next curse even before the Ashen and Sunken Sorrow curses so I've been wrong two times already :P
I'm not sure this can be considered a curse, to my understanding you become a ghost upon death as you get to the Sea of the Damned, which is why we do look like ghost in the Ferry, then the Ferryman makes us alive again and we get back to our usual look. But there are multiple ghosts in the main world, both random encounters and important characters like Ramsey, how did they become a ghost?
To my understanding there are multiple ways to exit the Sea of the Damned, the Ferryman is only one of them, and while most pirates wander around the Sea of the Damned for a lot of time before the Ferryman bring them back to life, making them forget all the time passed there, some Pirates, like the Pirate Lord himself, were able to escape this realm. This is still not very clear but it has to do with the veil.
We don't know how the Pirate Lord got out, possibly he made some important discoveries or got helped by someone, maybe the Ancients? or possibly even Belle, we really don't know that much about this character apart she's a Sea of the Damned expert.
We know the Dark Brethren is in posses of a Staff made out of a trident and the Gold Hoarder skull which has the power to open gates in the Veil, allowing them to go from the Sea of Thieves to the Sea of the Damned and vice versa at will, possibly using the power of the eyes of the Gold hoarder which are 2 veil stones. (Also, the box of wondrous secrets allows people form the Sea of the Damned and the Sea of Thieves to communicate with each other, technically creating a link between the two worlds, I'm not sure how. Only light and sound might go through, so it's like a videocall, but they might fit some small objects through the box, like a letter for example).
Another way to get in and out in fact is using the Veil of the Ancients (the mask) thanks to the power of the Veil Stones.
Basically, in order to acquire this curse we'd need to die and then get out of the Sea of the Damned from somewhere else, bypassing the Ferryman.
This is also likely to be released as they literally already have the Ghost visual effect, two different ones actually: the one we get in the Ferry and the one NPCs have. Even tho they had this effect ready for a lot of time, they still haven't released it as a curse, I expected it to come out with the Legend of the Veil update but looks like I was wrong. They either want to keep this exclusive to NPCs or have plans for the future. Only time will tell.
OCEAN CRAWLER CURSE
This is the +1, humans cursed by the power of a Siren gem.
I don't think they will ever make this for players tbh as I don't think a lot of players would be interested in it. Anyway, they have two ways of doing this:
- Somehow adapting it to a humanoid form, making coral and stuff grow on clothes etc...
- Actually change the pirate body, you wouldn't use clothes as cosmetics but Ocean Crawler's body parts, like different crustacean/coral like heads, hands and legs. If they'll ever introduce a merfolk faction and expansion, which I hope we'll get, that might be the time we get something like this.
These were the 3 curses I think have the highest chances to be added to the game next, the next two are really just a stretch:
There are 2 more curses I can think of, you are probably wondering which curses am I talking about, right? It's actually simpler than you think: we've seen Ashen and Gold skeletons, but there are also Coral Skeletons (which we can relate to Ocean Crawlers), Plant Skeletons and Shadow Skeletons! We can safely assume that all the curses skeletons have can also be casted on humans so Plant curse and Shadow curse might come to the game one day.
Unfortunately we don't have any information on how these skeletons got cursed but may have to do with Ancient magic too. I don't think there's a lot of chances that we'll ever get these curses, but we got Ashen and Sunken Sorrow curses out of nowhere so the next curse might be an unexpected one too.
I've talked about all the stuff that makes at least some sense, but as said we got some curses out of nowhere so we can speculate a bit more, even tho I'll get to stuff that is VERY unlikely to happen.
We know of three cursed chests (there's technically a fourth one we don't really know much aobut it): the Chest of rage, that we can link to Ashen curse, and the Chest of Sorrow, that we can link to Sunken Sorrow curse, so what about the Thousand Grogs? It's unlikely we'll ever get this curse, specially because I have no idea of how it would look.
Sirens are technically cursed Merfolk, this curse seems to not have any effects on humans tho.
One last thing I want to bring to the table, even tho being very unlikely, is us turning into Merfolk! All the following is just and idea so feel free to end your reading here if you were just interested in the speculation.
We can assume Merfolk are not humans transformed into fish-like creatures simply because the only civilization capable of doing so were the Ancients and there's no hint in the story pointing towards this, the way Ancients and Merfolk interact makes pretty clear these are two different species, but we know of one human that got turned into a Merfolk thanks to the power of gemstones, we also know that didn't end well as a lot of merfolk turned into sirens... but hey! We might use some other kind of magic to turn ourselves into Merfolk in the future! It could be a really interesting mechanic that changes the color of your skin and lower part of the body making you look like a Merfolk upon touching the water, also giving you infinite breath in underwater areas with a lot of oxygen (like the Shrines) and a little boost to speed in water (not anything too broken). This would be acquired through a Tall Tale that introduces the Merfolk underwater outposts, meaning this will be easy to get in order to not put some players in a disadvantage. Merfolk outposts might offer tail and skin (literal skin) cosmetics for the Merfolk form and Merfolk themed cosmetics for everything else.