My Deltarune Predictions:
Gaster is the Knight, but the Knight being set up as the potential main villain is a diversion and Gaster is actually a good guy.
Ralsei has ulterior motives, either he's evil or he's somewhat neutral (as in not as good as we thought but not a full-on bad guy).
This isn't one of my more likely predictions and I don't think it'll happen but I could see it being true, Mike might be the next Secret Boss.
In the final chapters, some of the Undertale character's will have more relevance on the plot, especially Sans, but for most of the game they'll be pretty minor.
And for Snowgrave Route:
Each chapter you'll kill more and more of the classmates.
You'll eventually fight Susie and probably also Ralsei.
You'll kill everyone in the Dark World and the Light World by the end.
Might have the Sans fight (or potentially a revamped version), but doubtful.
Even if Sans isn't a fight again, I think there's a slight chance we'll also get yet another version of Megalovania.