i know i created this thread over 2 weeks ago but I think it is not something about the leitmotif, if somehow each shadow crystal boss follows this rising and downfall cycle (carol being the knight could also follow this with rising being going up to mayor, downfall being after she lost dess or other things) then i think asgore could be one of the secret bosses in the next chapters as he had a good family but then the "downfall" is he got divorced, fired, and now lives in debt
this only works if the assumption that each secret boss follows the rising and downfall cycle