I had been wary about rebooting the series, but after that video, I'm thinking that maybe it would be possible to tell the same general story again in a different way with different actors.
In many ways, that song was similar to the "Once" musical episode, in that the characters were basically singing their backstories and basic motivations.
That got me wondering how a full "Once" musical could be adapted. At first, I was thinking maybe Act 1 would be Season 1 and Act 2 would be a mix of the subsequent seasons, but the subsequent season plots were so random that it wouldn't work (even Season 2 wouldn't work as a coherent and satisfying Act 2).
But if a musical consisted only of Season 1, the final musical number would basically be the Happily Ever After, with everyone re-united and remembering again, making for a more complete story.
The other challenge would be to incorporate Hook into Season 1, and also "redeeming" Regina and Rumple in one musical instead of seven seasons of 22-23 episodes.
I was thinking that if the Season 1 story was a musical, it could start the same way with Emma coming to Storybrooke. They could amalgamate Graham's role with Hook. So Hook would also be Cursed, and he would be the Sheriff that Emma slowly grows to care for. By having Hook also be the Huntsman who saves Snow White, we would more quickly be able to root for he and Emma to eventually be together. Of course, Hook wouldn't die like Graham, but maybe Emma thinks he died at the end of Act 1. That would also make Regina's redemption more believable since she doesn't murder anyone outright. Maybe she simply imprisons Hook to make Emma think he died. Or she blackmails him into pretending he dies, and he would also be struggling with his desire to seek revenge on Rumple. In this simpler musical story, Hook would give up his revenge for Emma in this Season 1 story.
Regina could be redeemed without all the murderous backstories. It would be more believable that maybe being a mother to Henry actually did change her. At the end of the musical after the apple turnover incident, she could decide herself to reunite Snow and Charming, or maybe she tells Emma that a kiss would wake Henry up. Her sacrifice would be realizing that Henry would be better off with Emma. Regina wouldn't be friends per se with everyone at the end, but she would have a clear character arc and the audience could still be "happy" she learned her lesson.
Then, there's Rumple. If Rumple were also to be "redeemed" in a "happy" ending, I think he would basically realize that Henry is all he has linked to Baelfire, and realize that he needed to change his ways. It would be too busy to introduce an adult Neal, so I think in a musical, Rumple could talk to Emma and deduce that Henry's father is Baelfire and Emma tells him that "Neal" died, and that makes him re-think his dastardly plans to bring magic back. Belle could help in his rehabilitation.
I guess there might also be a push to include Zelena, but I think there would already be too many characters to deal with.