Maeve has actually been with Outlander for some time as body double for Caitriona. There's no evidence or even a hint that she's been cast in a particular role, and if she were, it will be announced by Starz, or Ron, or Maril. Not by a crew member.
Remember waiting three or fours years for the next book? Or years to hear a TV series would be made? Or months to learn who would play our favorite characters? Droughtlander is difficult, but let's be patient and not give into the gossip that sites are pushing as 'news'.