quick lore speculations i have:

- December Holiday is the Chara of Deltarune. Dess is the one who "discards" your vessel at the beginning of the game and "no-one can choose who they are in this world" is the thematic "kill or be killed" of Deltarune. Your goal (on the normal route) is to defy this notion, which the plot is building up to with Susie and Gerson.

- The beginning of chapter 2 is a flashback (where Toriel says "Is that a knife? NOOOO". Kris used their knife to hurt someone (or defend themselves) in their past.

- "Determination" wordplay continues in Deltarune. "Determination" could mean the capability to choose an outcome or see things how they really are. The anxiety the fun gang have around the prophecy also align with the philosophical idea of determinism, where free will does not exist and everything that happened in the past, present, and future is unavoidable.

- Darkness not only serves as a simple lack of light in the game, but is also a source of INdeterminism, where everything is far more unpredictable, and with enough darkness reality itself can start to break down, and this maybe have been the thing that transported Sans from Deltarune to Undertale.

- FRIEND is critical to the plot and will be a Schrodinger's cat. Many hilarious quantum physics hijinks ensue.