

Zeniba and Yubaba theory
When Chihiro first gets to the top floor of the Bathhouse to meet Yubaba, there is a room with two identical doors. On the wall on top of them, there is what appears to be the coat of arms of the Bathhouse, with the kanji 油 (which normally means 'oil' but is used in the movie as a symbol of the Bathhouse) and Yubaba's bird clearly visible. So what does this mean?
I think the two doors represent the two sisters, Zeniba and Yubaba. It is a hint that Zeniba co-founded the Bathhouse with Yubaba. Another hint of this is that the first kanjis of the names Zeniba (銭婆) and Yubaba (湯婆婆) together form the word "baths" (銭湯).
But we know that Zeniba is not allowed to enter the Bathhouse. She had to resort to a trick to get in, and she got expelled when her trick was destroyed by Haku. So what's the logic behind this?
Here is the scenario that I came up with: Zeniba and Yubaba were once a single person. That person wanted to gain power over others, and to create the Bathhouse (which is basically the same thing in terms of what the Bathhouse represents in the movie). So, they made a contract with some unknown entity that granted them what they were looking for. BUT some precautions were taken so that they could not become too powerful. The person was to split themselves into two: one half would rule the Bathhouse, and the other would keep the magic seal with which the contract was signed. The seal represents the possibility of changing the terms of the contract, and gaining more power.
The greed and lust for power of the original person were inherited by Yubaba, the one ruling the Bathhouse. The other half, Zeniba, is what remains when all the passions of the original person are gone. That is why Zeniba is referred to as the older sister.
Zeniba is basically a bodhisattva: a person detached from all worldly things, looking at the world with only compassion. Meanwhile, the baby is the embodiment of Yubaba's worldly attachments, of her desire to protect what she has.
The reason why Zeniba is not allowed to enter the Bathhouse is that you need to be away from the world in order to be detached from everything. If she lived in the Bathhouse she would become greedy like Yubaba. So the door that represents Zeniba in the Bathhouse is a door that doesn't lead anywhere, it is just here for the sake of representation. Which is why Chihiro is instinctively drawn towards the other door.