Well, they might see different versions of characters from Undertale. For example, the RWBY characters could be really surprised by the unique battle mechanics in Undertale's AU multiverse. Maybe Ruby would be especially interested in the different types of weapons or magic used in that world.
Blake could also be a great character to focus on. Given her own complex backstory and her tendency to be more introspective, she might have deeper insights into the characters and situations in the Undertale AUs. She could form unique relationships with some of the more morally ambiguous characters in the Undertale multiverse.
One cool plot twist could be that in one of the multiverse scenarios, their enemies are actually their allies. This would completely turn their understanding of good and evil on its head and force them to reevaluate their relationships. It could lead to some really intense character development as they try to figure out what this means for their own world.
Well, if Undertale could watch Undertale fanfiction, it could lead to some chaos. For example, Sans might make jokes about the wild scenarios in the fanfics. Papyrus, on the other hand, might be eager to correct any misinformation about himself. And Flowey could use it as a way to manipulate things further, seeing how the fans view the characters and using that knowledge to his advantage in this strange, meta - situation.
Ruby herself could have major revelations. Since she's the main character, seeing different versions of herself and different ways her story could have gone might make her question her own path and purpose.
You should begin with a clear idea of what makes your Undertale AU different. Develop interesting character backstories and interactions. And make sure the dialogue is engaging and fits the tone of the comic.
If Naruto watches the multiverse fanfiction, he could gain new perspectives. Maybe he'd see how different choices in other 'Naruto - like' universes led to different outcomes. This could inspire him to think about his own choices in his world, and perhaps even consider alternative ways of doing things in his ninja career.
Well, it could be really interesting. Deltarune might find the different interpretations in the fanfiction quite fascinating. It could see new takes on the characters it might be related to from Undertale. Maybe it would be inspired by the creativity of the fanfic writers.
Well, he might get some really wild ideas. He could start imagining all the different versions of himself in other universes, like a Harry Potter who's a Slytherin instead of a Gryffindor. It would probably be both exciting and a bit confusing for him.
Well, one interesting plot could be the characters from Class 1 A finding themselves in alternate universes where their quirks are completely different. For example, Deku might be in a world where everyone has no quirks but still manages to be a hero through his wits and determination.