Where/If Megalovania appears in Deltarune is a popular topic especially for fan songs. Most of the time it's given to Susie normally after she discovers Kris's lies and betrayal. However, I think Ralsei should get Megalovania.
If you do end up fighting Ralsei in the weird route they wouldn't really want to hurt Kris or Noelle knowing that they are unwilling participants. Their goal would not be to hurt their friends but instead get you, the player, so pissed off and annoyed you rage quit and stop trying. This would already make Ralsei a strong parallel to Sans at the end of the No Mercy route where he does something very similar.
Another tie to Sans is that Ralsei knows the game mechanics and most likely knows how to break them like dodging attacks or messing with the UI, And in Chapter 5 Ralsei is hinted to be holding back in some sense which would make them unexpectedly strong just like how Sans catches people off guard with how strong he seemingly is.
This would also countinue the character progression of learning to defy the prophecy in the weird route since they weren't able to get that progression from the chapter 5 dark world. They have to stand up to the angel to protect their friends, even if it means defying what they thought as unavoidable.
All these parallels to Sans would make giving Ralsei a Megalovania theme a no brainer imo.





















