We go and see the Pale Tree, who serves as a repository for the sylvari collective memory. This is to say, every experience a dream-bound sylvari has is collected up by the Pale Tree and stored, which includes Caithe's memories. Caithe's conversations with Glint are the tip of an iceberg of possible secrets held by the Pale Tree.
The Pale Tree will use her magic to allow us to re-live selective slices of Caithe's past, which will give us significant clues into what she is up to. Or not. The Pale Tree might actually steer us in another direction entirely to give Caithe the freedom she needs to carry out her plan, and we could meet up with her again much later.
What we saw in the trailer was also hinting at the Forgotten, the four-armed snake-folk who guarded Glint in the past. Our eventual destination might be a location hidden away of some significance to the Forgotten, perhaps also to the dwarven Brotherhood (and therefore the Zephyrites), where we meet Caithe with the egg... perhaps even Gleam? Well, that's enough speculation for now :)