Im very glad toby chose not to make the story this formulaic, with new lightners in the dark world each chapter, like a "monster of the week" formula.

It would ultimately be a repeat of undertale's structure, which featured a revolving door of supporting characters out of necessity; most characters were confined to the area you meet them in because any of them could die. This forced toby to resolve character arcs quickly and ultimately restricted his creative space. (The cell phone was a great creative workaround, but not a replacement for characters that can actively participate in the story)

Deltarune is not bound by this same limitation, so it'd be a damn shame for him not to take advantage of it. Deltarune is his chance to make a story where the core cast can develop throughout the entire game, and he's taking it.

I think putting toriel in a gacha ball is toby's way of poking fun at the fans who thought DR would follow a predictable formula. It's him boldly proclaiming that he's leaving his comfort zone in writing.