I think Tiger Lily could be Lucy’s fairy godmother, and could be in HH. I think she will be instrumental (cursed or not) in breaking the curse. As to how, I don’t really know.
I also think that Regina will need to approach Rumple again for help. Again, no idea how he could.
I have only one idea right now. Suddenly out of nowhere there will be a magical McGuffin that freezes time right at the moment when the curse starts to break. Then, someone with magic will do something else to save Henry while he is still alive.
I considered that Henry’s last breath would be saved shortly after the curse is broken (with Tremaine’s help?). But this would mean that someone would need to be sacrificed to wake him up maybe.
Keeper of Regina's bravery