Thematically, Knight Papyrus would be a very good twist, actually.

A lot of people dismiss him as a "joke candidate", a massive cop-out Toby would never put in the game. I think that couldn't be further from the truth.

In Undertale, Papyrus's biggest wish is becoming a Royal Guard, and getting showered by adoration from everyone because of it.

Isn't that exactly what being the Knight in Deltarune would entail? A brave hero, creator of the Dark World, adored by all of its inhabitants?

Darkners' "purpose" is to serve lightners. Ralsei demonstrates that this includes comforting them, affirming when needed. And if there's anything Papyrus seeks, it's affirmation.

In addition to his UT traits, we can try to "extrapolate" what he's like in Deltarune based on his oddly explicit lack of appearance:

• Doesn't leave his house - socially awkward.
• Asks Sans to find him some friends, then panicks and delays the first meeting - wishes for friends, but is unable to make any.

He'll likely meet the main cast on his own terms: in a Dark World, surrounded by darkners who affirm and strengthen him, ready for an epic battle as a Knight fighting against the Heroes of the Prophecy. But in the Light World, as just Papyrus? No, his hordes of friends need to know his cool and amazing side first!

In the Dark World, he's Undertale Papyrus's dream come true - cool, brave, awesome, adored by everybody. In the Light World, he doesn't even have the courage to leave his house. That's why he only goes out after dark, creating a fountain, having an adventure, and congratulating himself on what an amazing world he's just created.

He doesn't know about the Roaring - just like he didn't know that capturing Frisk would lead to their death. He's naive. And if Ralsei tries to explain that his actions could have disastrous consequences, he might even try to reject that possibility, as it would mean giving up on the only place where he feels comfortable.

He doesn't know that after he leaves the Dark World, his elected ruler becomes a tyrant, and all the inhabitants start rebellions. Isn't the story supposed to be over? Hasn't he defeated the monster, saved the kingdom, and given everyone a happily-ever-after?

Dark worlds are heavily tied to escapism, especially in chapter 2. I think that a more socially anxious version of Papyrus would unironically be the best character to explore that theme with.

(Not to mention, "royal guard" and "knight" are almost the same title. This parallels him really nicely with Undyne, too: Undyne is an actual battle-hardened Royal Guard - the real-world version of that would be a police officer. Papyrus, a wannabe over-idealized version of a Royal Guard, would instead be a fantasy knight in shining armor.)

EDIT: My copium levels are approaching critical, but check out OST 029 - Gallery. The song that plays DIRECTLY between BOTH Queen's and King's fight, and a leitmotif that doesn't appear anywhere else.

Four looping notes... It's not an exact match - that would be spotted and posted everywhere instantly - but don't they sound similar to Papyrus's theme, played backwards?