Random headcanons:

King's first name is Cleaver

Queen has the missing Queen of Spades concealed in her casing, she's sort of a composite Darkner

Ther was a real merry-go-round toy in the classroom cupboard - someone put the Black Joker card in there on purpose in order to power both up

The Seam we meet is the Red Joker card, and the stuffed cat toy is his shop. Seam looks like the toy because of association magic.

Blue Birds are symbols of obstruction

If Rouxls Kaard can somehow bind with a more detailed manual or manuscript, he could attain unfathomable power

Dogs will be significantly important at some point in the narrative

The Silver Watch is larger in the Dark World and worn like a champion's belt

A real-life sword would turn into something like a fighter jet or super car in a Dark World, assuming that it doesn't become an intelligent Darkner

The essence of Darkness that pours from a Fountain is bound into the earth itself, so if you scoop up a good amount of soil, you can create a short-term Dark Fountain from it even if it's raised above ground or stored inside something. One the earthen matter ruins out, it will disintegrate until restored to the planet's crust.

Plants will have a very special relationship with dark fountains, and the way plants draw nourishment from soil will relate to this as well as a light/dark metaphor vis-a-vis their leaves.

Jackenstein will be a lot more powerful in a different fountain

Addisons used to live in TV Land, but the decline of television migrated them all to the Cyber City.

Noelle has a cute upper lip

Queen is the only black-coded character in Deltarune

Gerson shrunk a lot with old age

There's a disturbing lack of snails in Hometown

Kris's sex will never be important, and this is the only reason why he's a they/them