Maybe this has been discussed before, but I was thinking about the prophecy.
What if Susie not only isn't the girl in the prophecy, but is perhaps supposed to be a villain?
Think about it. One of the core themes of Deltarune is destiny, prophecy, determinism, and then those that try to break out of it. All the secret bosses are obsessed with freedom and breaking away from set paths, while Ralsei knows everything that will happen because of the prophecy that must be followed. Meanwhile, Susie's whole schtick is that she felt obligated to be the "bad girl" no one likes, as if that was her destiny, and all the adventures she's had now have included her breaking out of this deterministic destiny to instead do what she wants to. It's also why Gerson focuses so hard on teaching her that the prophecy doesn't matter - the final panel somehow indicates that she's still supposed to be bad. This also then makes it interesting that Ralsei regularly talks about how nice she is, and tries to shield her from reading about the prophecy, all while Ch.4. has so many moments wherein she talks about how great it is to be part of the prophecy.
Or maybe I'm a crackhead