I think they could do next season about Snow and Charming trying to get the people back on Snow’s side and get her kingdom back. They could end with Snow and Charming getting married or they could even hold that out longer if they wanted.
And they could do another season the time between that in the curse. There is no reason that there can’t be years between the events of Charming kissing Snow at the castle and the curse being enacted. They could fit a lot in there if they want.