I think this is all setting Henry up to be a big hero in the show, especially given how he’s tired of reading about heroes. If the what the seer said is something like him going evil or something maybe he will be one of those people who are able to change their stars. This may have been foreshadowed last session when he tore the ending out of the book; i know it was done for a practical purpose but it could have deeper meaning.
I doubt it had anything to do with Henry becoming the nest dark one. it seems like whatever the seer said, Rumple originally thought it was Bae, but if it was about the dark one, I don’t think Rumple would brink that sort of darkness around him…not if he had forewarning.