WMG: Undertale

Toriel is the former Queen of the monsters

We know there are "races" of monsters, because you can exhaust all the monsters in an area by killing them all. The silhouette of the king of the monsters is bipedal, has feet like Toriel, and horns, and is bigger than other monsters, just like her; so is probably another of her race. The monsters all attack you, even when they don't want to, and some plan is in the offing that needs the souls of human children. The only place where human children come from is through these ruins, and the one overseeing the ruins area is Toriel. Toriel has been acting deliberately to prevent the childrens-souls plan from happening, but the monsters don't do anything about it; in fact, they're intimidated by her, and she's a very powerful and skilled spellcaster.

So the theory is: during the war, Toriel is wife of the king of the monsters. Bad things happen, and the monsters get kicked downstairs. The king hatches a plan to get out, involving the souls of human kids. Toriel opposes this, so volunteers to be the one to oversee the ruins and entice children in by pretending to be nice. The kicker is that she actually IS nice and has simply been failing to carry through on her own desires. And if perhaps the king and queen lost a child or two in the war, it would explain both why the king has gone for such a malevolent plan after presumed ages of peace (perhaps with influence by a certain vegetative person), and why Toriel is... Toriel.

If the above WMG is true, Asgore will be An Ice Person to contrast with Toriel's Playing with Fire.
  • Alternatively, he will also be Playing with Fire, but his Personality Powers symbolism will be the opposite of Toriel's, revolving around fire's chaotic and destructive nature rather than its nurturing warmth.

If the player spares Toriel, he goes on a quest to make outside the ruins safer in hopes of coping Toriel to leave them, as well.
  • This could lead to her reacting 3 ways, from being happy of taming said monsters (no kill run) to horrified on what the player character does (kill all monsters).

Toriel destroys the one-way door to the outside once the player character leaves. Alternatively, if Toriel is dead, the ruins collapse with no caretaker maintaining it.

There will be various caretakers in each part of the underworld.
  • Seems to make sense, especially when it could be possible that Papyrus/sans could be dual caretakers of their own domain.

The red text in the bad ending of the demo and the meaning of the protagonist being "determined" and "finishing the job"
S/He's decided that they're going to kill all of the monsters that have been sealed underground, which effectively brings an end to the war shown in the introduction of the game.

New Game+ in the complete version will be an entirely different story
Flowey, now knowing that the protagonist succeeds, will do everything in his power to stop him. Assuming you didn't work together with him.

The concept of determination is a meta-nod to the actual player
Determination, or variations thereof, keep popping out when you save. Flowey mentions that "it is the power to shape the world by sheer force of will." The player is actually being referred to here, as that person can be the one who can know the full outcome via walkthrough or perusing through forums. Whatever s/he wants to do, s/he can do it, as long as they have the determination to reload at that critical moment and start from that point on.

Continuing the meta: In the bad path, an entity has taken control away from the player.
When you've met the conditions to simply be "determined" and look in the mirror in Toriel's house, where it would normally say "It's you!", it instead reads "It's me, <name>." It's not the player anymore, it's the protagonist. But then who is the player character, and what's the context for the player now?

Flowey: In fact... You're <name>, right? We're still inseparable, after all these years. Listen. I have a plan to become all powerful. Even more powerful than you and your stolen soul.

The "stolen soul" Flowey refers to is not just the character you play, but you. <name> is an acquaintance of Flowey's. You named the character that way, played the character that way, because you're not in control anymore. After all, you wouldn't normally kill everyone you meet, would you?

The protagonist is literally a 'fallen human'.

The phrasing used for the naming screen is very precise. They didn't just fall from Mt. Ebott, they fell in another sense. And there's no real indication that it's only the player's control that gives them the ability to SAVE...