Sorry if this is already suggested.
Plant curse - replaces hair with plants, could add some extra bits on the body, like golden eyes or a pigs snout or feather sticking out :D to make it seem like its a merchant curse
Curse of the deep - One hand a kraken tentacle, snake like yellow eyes (kraken set eye patch for inspiration)
Curse of the frozen horizon/terror - Kind of an obvious one, some body parts appear as frozen or similar to golden curse (patches on your body)
Curse of castaway - Not sure how to pull it off in sea of thieves friendly style exactly (im not an artist) but something very similar to Bootstrap Bill Turner from POTC (Barnacles, moss etc)
Curse of the mermaid - Eyes and middle of the chest have a gem in them, maybe it could be first curse that has variables which you can choose/unlock (gree,blue,red)
Do you guys have cool ideas for curses? Apart from Skeleton which everyone who liked POTC has already suggested ^^
edit - corrected the nonsense i wrote while at work.