So everyone knows the theories about how Susie's importance to the story may be a defiance of the prophecy (As she is fairly consistently shown breaking rules and status quos that other characters just don't, among other reasons). That she wasn't *supposed* to be "the girl", or that she wasn't *supposed* to be anything more than "The Dragon" from dragon blazers, a threat only stated to be present in chapter 1 of Lord of the Hammer by Gerson.

In the chapter 3 egg room, the Forgotten Man talks to Kris about a different time, a time when the world looked different. In this order:  
"WELL, THE WORLD CHANGED. SOCIETY WAS DEVELOPED"  
"THE EARTH WAS COVERED IN WATER, DINOSAURS APPEARED, AN ICE AGE"  
"HM... ACTUALLY, THERE ARE STILL DINOSAURS... IS IT MIXED UP?"  
"IN ANY CASE, THIS ISLAND IS THE ONLY PLACE LEFT I CAN TALK"  
"DO YOU UNDERSTAND WHAT I'M SAYING?"

ok so there's a lot I want to talk about here. The forgotten man *seems* to be talking about deep time of real life with the order of "the earth was covered in water > dinosaurs appeared > an ice age" but before all of this he says that society was developed. He also skips over many other significant events afterwards, such as the appearance of land animals, extinction of dinosaurs, etc.

I think whatever this is, it implies the world of deltarune has a similar albeit flipped origin to our own, with "society" developing before dinosaurs appeared. With the anthropomorphic monsters in UT/DR I don't think it's a stretch to say he also doesn't mean "dinosaurs" in the traditional sense that it would mean in our own world.

I may be starting to grasp at straws here but wait... He says "THERE ARE STILL DINOSAURS... IS IT MIXED UP?" I believe he's either talking about Susie specifically, or all of us wannabe prophecy rebels, as "dinosaurs"  
If Susie is the "dragon" (Which, obviously a dinosaur can be likened to a dragon and vice versa) and she defies the prophecy by also being "the girl", that sounds to me like it's being represented by her being the only living dinosaur.

This string is more loose, but the forgotten man also says "IN ANY CASE, THIS ISLAND IS THE ONLY PLACE LEFT I CAN TALK". One of the most common tropes for stories about dinosaurs still living, almost always involves islands. Susie's parents *have* been mentioned to exist by Toriel and probably other times that I can't recall. They are definitely Chekhov's gun, as are Kris's biological parents imo, but that's for a whole different theory. Could it be that Susie's parents on this either literal or metaphorical "island" that Susie is a relic of?

Also I don't know how to connect this but the Forgotten Man's warbled description of natural history "THE EARTH WAS COVERED IN WATER, DINOSAURS APPEARED, AN ICE AGE," sounds like it could be alluding to The Roaring. Of course there's the fact that darkness is CONSTANTLY likened to water in this game, but think of it like this, what if:  
"the earth was covered in water > dinosaurs appeared > an ice age"  
=  
"the earth was covered in dark fountains > titans appeared > an eternal darkness"  


Inklings of a theory?...