This is an alternate take on 5A. It holds to established canon ships.
Regina begs Emma to save Robin. This is the big one, the first request, the one that breaks the heroes’ insistence that they are not going to have Emma use her magic because the darkness just too seductive. This is the episode that made me buy that they were going there with Emma, that they were really going to make her family partially responsible for her darkness.
The dwarves (they WANTED to be a part of the adventure, after all) find out that the brother they thought was dead might actually be in this realm, nearby, and that he might have something they need to release Merlin from the tree. But their brother might be in more trouble than they thought, and it seems like they might need to make a deal with the Dark One to help him…
David and Arthur go on their bro-quest for the magic toadstool, but those left behind get a tip from Lancelot that all may not be right in Camelot. They discover that Guinevere is enchanted with the Sands of Avalon and attempt to release her from the curse so she can help them. (Perhaps it turns out the Sands of Avalon, as the creation of a former DO, are powerful magic that can only be undone by another Dark One’s magic.)
Merida makes a deal with Emma for the spell to change her fate so she can rescue her brothers. Belle still convinces Merida she can accomplish her goal without the spell.
Henry’s date with Violet goes exactly the same way, and for the same reasons–Emma is already losing perspective on her darkness. After they free Merlin, she offers to fix it for him–but Henry, as a true hero, turns her down (leading Emma to tell him that everyone else let her down, but not him.)
Arthur and Zelena work together to trap Merlin by binding him to Excalibur. Killian gets hurt in the rescue.
We find out that Killian and Emma have been planning to move in together. Instead of Emma admitting she wants to move forward with her relationship with Hook, she admits that she’s having doubts and that she hasn’t been honest with him because she loves him and doesn’t want to hurt him. He says that he’ll wait until she’s ready. This honesty unlocks the Promethean Flame, but Killian collapses just like before and Emma tethers him to the sword to save his life, realizing that she does want that future after all–at any cost. And why is everyone so judgmental about this? Doesn’t she deserve to be happy? Haven’t they been the ones prioritizing themselves over her this whole time?
And that’s how you turn Emma dark.
Then, you have her working to undo making Hook a DO without killing him, which involves sacrificing someone to the darkness. You have her working to bring this plan to fruition. You have Hook succumbing to the darkness without knowing what’s going on–maybe he starts working with Emma because he is tricked by her arguments about doing what’s necessary, because he believes her. Maybe he turns cruel without meaning to. Basically, you show the influence of the darkness in him before he gets his memories back.
And then the end of 5A plays out largely the same, except Emma does have to sacrifice herself to get rid of the darkness. She dies and Hook is left de-Dark One-ed… and Hades comes and offers him/everyone else a chance to get her back. You see, he’s looking for something to use against his brother, who cursed him to the Underworld, and if they help him, he’ll help them. After all, he’s the ruler of the Underworld, and he’s never met a rule he can’t bend, even that pesky one about not bringing anyone back from the dead.
Rumple doesn’t become the DO again right then, but he offers to help them navigate the Underworld because he wants the power back, because Belle’s gone and he might as well. He cautions them not to trust Hades, because his games are always rigged.
They descend into the Underworld.