I always saw Once as being like a mature version of Kingdom Hearts with a more realistic take of Kingdom Hearts' concepts of light and darkness, the crossing over of Disney characters and their worlds with modern day people and settings, as well as the use of possessing hearts among other similar elements.
As such, I really appreciated Once making somewhat indirect references specifically towards Kingdom Hearts. Season 4 made some nice references to the series with the doors that the Frozen characters and The Apprentice & Isaac went through very reminiscent of the doors seen in the Dive to the Heart tutorials and the Door to Darkness itself featured in the game.
Henry using the key to get into the storybook was also a nice reference to the Keyblades used in the series. His status as the author and overall role and persona in the series is reminiscent of Kingdom Hearts' main protagonist Sora, the pen notwithstanding being like a Keyblade itself with its "unlocking" powers.
I also really appreciated Cerberus in Labour or Love because it brought even more Kingdom Hearts vibes. Despite being a character from Hercules, and Greek Mythology, Cerberus has been featured prominently in many of the games in the series. Him being featured actually felt more like it was a reference/adaptation of Kingdom Hearts instead of Hercules, especially with Herc, Meg and Snow teaming up to defeat him. Reminded me of the trio of Sora,Donald and Goofy going up against him. Also liked how Herc and Meg initially died to such a hard "boss" like most of us Kingdom Hearts fans have or experienced