I'm not sure if this is the sort of stuff I should be putting on here but I had some theories for the Secret Commonwealth that I wanted to share.
So in La Belle Sauvage there was that enchanted island where the river giant and those forgotten people were. That was never fully explained and I think that could link to the next book.
Another thing that happened was that Malcolm (or maybe someone else, I don't remember too well) looked into the Thames and saw some lights under water. Someone later described that as the secret commonwealth.
We know that in the book Lyra will be travelling through the Levant. In Lyra's Oxford there is a catalogue for a trip on the S.S Zenobia (thanks to Amyosaurus for reminding me of this!) and there is a note on it written by Lyra saying she will meet someone. The S.S Zenobia cruises are oriental and Levantine. These 2 parts of the catalogue lead me to believe that this is related to the Secret Commonwealth.
If anyone has any idea about these theories let me know!