One popular theory is that Susie will be the final boss of the weird route with a new rendition of Megalovania as the theme. What do you think?
People are already joking about how Toby might encounter a Gaster fun value in the game during the stream only to then skip it and never acknowledge it at all. It'd be a really sad yet also funny way of saying that Gaster is not getting an official mention, ever. (Unless he ends up having an important role in deltarune)
While Susie being the final boss of the weird route would be pretty cool (and definitely heartbreaking!) I don't really want Megalovania to show up again. It's already had its time in the sun with the Halloween Hack, Homestuck and being known as Sans' fight song. I'd rather we grow something new out of, say, Vs Susie and Hammer of Justice or the like.
I wouldn't put it outside the realm of possibility that the stream involves Toby and crew playing with fun values, since we did see a bit of that sort of teasing in the chapter one stream featuring Nubert and the annoying dog with maracas. I don't know if any fun value stuff should be taken as gospel involving Gaster's role in Deltarune, though. I'm pretty certain that Gaster is going to be important to Deltarune no matter what.