Cordelia Foxx/Billie Dean Howard
Since Sarah Paulson's return has been locked in, we can assume one (or both!) of her characters from Murder House and Coven will return in some capacity. More promisingly, both of these women are alive. Last we see Cordelia, she's taking over as the next Supreme, and she's running Miss Robichaux's Academy. Billie Dean Howard pops up in season five, Hotel, so she's likely still around as well.
As for the dental appliances Ryan Murphy teased earlier in 2018? They could apply to these characters, or Paulson could be playing a new character as well. Don't forget: she previously doubled up in Hotel and Roanoke.