If next season is an AU then literally anything would be possible, but it would also mean throwing away all the character development of the last 6 seasons so I'll be surprised if it happens that way.
Even with the news of JMo leaving, I still have a difficult time believing they'd really kill Emma, especially since they want to leave the door open for JMo to return if she wanted to. I think it's more likely her fate is somehow unknown. I'm just not sure how they would do it. If Colin is leaving, then it's simple enough - they just sail away on the Jolly Roger, although that would be weird for Emma to leave Henry. I don't know.
If Robert Carlyle is staying, then maybe Rumple himself doesn't actually die, but the Dark One curse dies? Maybe Zelena dies? Or back to the previous idea that Regina dies and Lana returns as the Queen? If Emma were to be missing or comatose and Regina died, that could definitely traumatize Henry enough for him to be this cynical recluse they've been talking about. That still leaves the aging up issue with other characters though, unless Henry has been spending time in the Dark Realm.
Renewals and cancellations are expected to be announced before the end of the week, but even if the show is renewed, we still may not know who is leaving right away.