This theory may be a bit of a stretch, but it does have some evidence behind it, and I wouldn't put it past Toby to do something like this. Hear me out.

As we've seen from Chapters 1 and 2 of Deltarune, each chapter's main boss tends to have blue as their primary color. With Kris's main appearance in the Dark Worlds being blue also, it leads me to believe that they will be a main boss of a future chapter. While this is not a lot of evidence to the theory, we must also take into consideration another Deltarune prediction theory - the Chess Theory. This theory predicts that each chapter's main boss will correspond to a different chess piece - King, then Queen, then Rook, then Bishop, then, in Chapter 5, a knight. Since it is already widely believed that the Knight is Kris, it makes sense for Kris to be a future chapter's boss. But this color choice may just be a coincidence, as blue is a very common color to design a character off of. But what if I told you there was another example of this?

Let's move on to the secret bosses. At a glance, each secret boss tends to have purple/magenta as their primary color and yellow as their secondary color. Who else has these colors, and who else have we theorized will be a future secret boss? None other than Susie. Her Dark World appearance uses magenta as a primary color and yellow on her arms and belt as a secondary color. Toby Fox knew exactly what he was doing with these character designs.

Thanks for reading, and feel free to leave your thoughts in the comments!