On the season four finale of Once Upon A Time, The heroes and villains had roles reversal when The Author rewrote history for a dying Rumpelstiltskin. While it was a interesting change rather than another cursed (God, How Many CURSES did we have?), The final fifteen minutes in Part two was what really got me going OH MY GOD!
(SPOILER ALERT> STOP IF YOU HAVEN'T SEEN THIS EPISODE)
After Henry, the biological son of Emma and Regina's adopted son, save the day by becoming the new author and reverses Harold Teller's twisted version of the Enchanted Forest, We get a look at nearly dead Mr. Gold who just became unconscious. Emma and The Operation Mongoose team along with The Apprentice figure out something. If Gold dies, The dark one needs a new host. They attempt to capture the dark spirit in Yen Sid's hat but it breaks free and possesses The Apprentice. Emma uses her light powers to release the mage before the hold became permanent. After the Dark One Spirit is released in the streets, It tries to capture Regina since she is the perfect choice. Emma, not wanting Regina's Happy Ending to be ruined, tells her family to figure out how to stop HER and sacrifices herself and the final shot is of the dagger with EMMA SWAN on it.
Now....You're asking: How does Kingdom Hearts relate to this?
Well, Let's go back to Kingdom Hearts Birth by Sleep and the original Kingdom Hearts. In Birth By Sleep, Aqua, one of the three characters you control, is trying to save both her friends Terra and Ventus from Master Xehanort. Xehanort, who was manipulating events in the games, had already stolen the body of Terra and his cohort, Vanitas was about to reunited the legendary X(Chi) blade to open Kingdom Hearts. After defeating Vanitas, Aqua seals away Ventus in the new Castle Oblivion and fights Xehanort in the ruins of Radiant Garden. After the fight, Xehanort and Terra falls into the realm of darkness. Aqua goes after them trying to save Terra's body but with the gate closing, she sends terra with her armor and keyblade thus sacrificing herself to darkness.
The visuals of Emma absorbing the dark spirit is very much like the scene from KH1 where Sora meets Riku on the pier during the heartless attack. (See video...Sorry for the horrible quality)
Even the possession scene is very Kingdom Hearts!
So, My thought was Will we be seeing Season 5 as The Kingdom Hearts season. It would make sense since there is a lot of world/dimension jumping in this show. There is also the fact that Season four dropped hints using.....DOORS as portals to Arendale, The Enchanted Forest, etc. The Key and The door page were a major hint.
Another clue is the Storybook and the library of empty books. In Kingdom Heart X(Chi) and Kingdom Hearts 2.5 HD Remix, there is mention of a Book of Prophecy that can foretell future events in the Kingdom Hearts universe. The storybook does the same thing by telling the character how story gets played out.
Why Kingdom Hearts would make Season 5 HOT?
Once Upon a Time is already taking classic fairy tales/Disney films and making them more interesting. Kingdom Hearts does that by adding Final Fantasy aspects and deep story lines. Imagine if these worlds collided? Imagine a massive Heartless invasion in Storybrooke?
We could also see Henry getting much bigger role than Season four. In the same finale, Henry had a conversation with The Apprentice about being an author. Henry proved himself worthy being destroying the magic pen stating "No one should have this much power" The Apprentice smiles and says "I think we found the right person for the job" What did he mean by that? Could Henry team up with Sora, Riku and Kiari? Season Five should be more Henry-focused. Emma is captured by The Dark One and Henry will stop at nothing to get his mother back.
Rumple might get redemption and connect with Riku about being freed from darkness. Imagine Rumple and Riku fights..
Secondly, Kingdom Hearts III is coming soon. There already an announcement of Kingdom Hearts Unchained X for mobile devices and Tetsuya Nomura, Kingdom Hearts director, is hinting a major announcement at E3 2015. Having Kingdom Hearts in Once Upon A Time would be a major promotion for the game. Who know if might see Storybrooke as a world in Kingdom Hearts III? Here's hoping!