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Anyone investigating after the Ritual is complete had better bring a backhoe. Or a dwarven digging team. Unless the PC's left Vetra Kali alive in the horn (bad idea) the whole place collapses to rubble.
Between 2 and 3 the PC's get to hang out in Ghastenhall for a month. Aside from the things already in Book 3, you could do almost any urban themed adventure. If they are playing different characters that opens things up
If you have the entire series you could do things that foreshadow upcoming events. A group of ice elf explorers find (and are wiped out by) Thorn's palace in the north (or the Cairn Linnorm). A small party on the mainland working with Cedric recruiting for the army that's needed in book 6. A group of thieves to do a B&E on the palace in the capital. If you want to stay evil, there are many things you could do with a group of duergar raiders (the PCs have to find their home while setting up for The Vale).
There are several "loose threads" in most of the books. The tomb of an Ice Elf king is mentioned in book 5. An aquatic dragon steals the horde of another dragon before the PCs can take it. There is a settlement of Tritons that needs exterminating off the eastern coast. I can't remember them all (my group is rapidly approaching the last act of book 6).