Potentially unpopular opinion here but... I don't think Hook will be the (only) one to save Emma from the dark side. I think Regina is going to play a part in that. And no, not because of "fan service", but because it makes sense.
Regina is the one who has experience with magic, and especially with dark magic and going dark in general. Killian admits that he tends to try and avoid magic whenever he can. Yes, he supports Emma and her magic, but it's very clear that it's an exception because it's Emma. Which says a lot about how much he loves her, enough that even his (very justified, long-held) negative view of magic is flipped in her specific case. But he doesn't actually understand it. Not like Elsa and Regina can. And we already saw Regina step up to defend or protect Emma and her magic in 4a, when she laid into Snow and David about letting Emma get rid of her magic.
I do think Killian will be around to support Emma and try to get through to her. He's a light in her life right now, and she'll need that. But I'm honestly kind of hoping that we'll see some help from Regina as well. It's about time she started helping, especially Emma and the Charmings, because she does owe them. And also, for her personally, it would be a big step in the right direction because it'd mean taking responsibility and doing good in a selfless way, which are two things she really really needs to learn.
I think it's very possible that we'll see Regina in the role of (reluctant, possibly slightly snarky) mentor/parent/friend, with actual practical advice, and Killian as part of Emma's support system on an emotional level. Killian is a lot of things, but he can't be Emma's mentor in this. He can't, and shouldn't, be her everything. And we've already seen that while Snowing love their daughter, they have no real clue how to deal with the magical aspect of her. With Elsa gone, Regina is the only one who can realistically understand and help Emma in that regard.
I know even the idea of Emma and Regina being friends and working together is enough to cause drama and offense and upset, but I'm pretty sure it's going to happen. And I'm sorry, but it's not fan service, any more than the relationship between Hook and Emma is fan service. From a storytelling point of view, it simply makes a lot of sense.
And another thing. A big part of the reason why Hook and Emma make sense together is that they have similar backgrounds and understand each other. They both have that bit of darkness, they both have done things they're not proud of. Emma is many things, but she's not an innocent precious flower, she's a convicted felon (who by the way still drives around in a stolen car).
And that same thing is the reason why Emma and Regina understand each other. They both know what it's like to struggle with darkness, to be rejected and alone. Different reasons and different results and different choices, yes, but the same goes for Emma and Hook. Emma has always maintained that light inside her and inspired others to seek it out in themselves. And I think now it's time for those others to pay her back in kind. Regina owes her that, and Hook owes her that, so to me it makes total sense that it'll be the two of them who help Emma out with this.
This is all speculation, obviously. But this is what makes sense to me, and honestly, it's what I'd like to see.