Good to see a Zeit campaign going well. Let me see what I can suggest.
Reading ahead in Zeitgeist is often very rewarding when it comes to working in player backstories and themes. Do keep that in mind.
Dalton
-Other posters have mentioned Dalton's sister Terakalir. but it is also worth noting that the gnolls of the Isla dola Focas would regard Dalton as a near-god, once they find out about his heritage. That should affect how, say, Minister Pardo treats him in Book 5.
-In later books, Pemberton gets effectively promoted to party ally, and crucial to saving the world. Do keep this in mind, don't play him as too villainous.
-If Dalton is big on gadgets, then he might be interested in Pemberton's duplicant technology. Act 3 has some further stats on it. In my game we invented 'complicants', duplicants without any combat power and instead used for communications purposes, allowing for instant communication across for long distances.
-Harkover the dragon knows and doesn't much like Pemberton, so it would be pretty neat to have Harkover try to turn Dalton against his further/turn Dalton to Harkover's side
Rachel Lang
-Can I assume you have read through Bonds of Forced Faith? There are a few things in there that might be relevant; her knowing Stanfield, and her knowing about the Bonds ritual. This ritual was retrieved from Ashima-Shimtu. It is also a bit of a 'forerunner' of the Sacrament ritual used to create Srasama, and used to empower other characters later in the AP.
-Ashima-Shimtu is a 'reformed' demon, even if it takes her quite some time to get around to the 'reformed' part. One of her first acts upon being freed is to spread the Sacrament ritual around, indirectly causing an immense number of deaths. Her interests would include acting against Nic, being set free, stopping the angels who want to kill her, stopping the Clergy from doing evil stuff, and investigating blood magic. Ashima-shimtu has quite a bit of blood magic, so you could create hooks relating to that and Ostea.
The Boy
-Kasvarina has a history of marrying and getting her husbands to do her bidding; read about Bhalu in Act 2.
-Book 9 has a lot on the Unseen Court, lots of hooks you could try and work in for the Boy. Unseen and Hedgehog Court members who recognize him and want to work him into their schemes.
-There could be some Vekeshi fellows who see the Boy, fey-trained and Elfaivaran-born, as a natural ally. Note that Kasvarina Varal was a founding member of the Vekeshi, and the Vekeshi assassins.
-Have you written much about the eladrin of the court? It could be that they are an enclave who fled Elfaivar pre or post-Malice, into the safety of the Dreaming.
-You could have Grappa know the Boy, having heard Kasvarina talk about the Boy when she still had her memory.
-Perhaps you could write in a weretiger from the Children of Hewanharimau who are interested in pre-emptively turning the Boy against the enclave eladrin.
Ashiar Gancho
-'Hear the spirits of machines' sounds very much like Tinker Oddcog, as you say. You could perhaps steer him towards the Gidim; read up on Book 10 and the start of Book 11, learning about godminds. In my game I had it that the godmind urn was a Gidim artifact, containing the remnants of a vast alien psyche collective.
-If Ashiar was secretly Ob, then you could work that into Book 8, where the Arc of Reida can be used to re-play his memories and learn more. But I think you'll need to be very careful to make sure that the player has some choice on these revelations, and on what comes next.
Lamorak
-Again, Book 8 has a ton of memories that might be relevant.
-Plenty of golems and walkers turn up in later books, like the draconic witchoil golems. Might be useful.
-Perhaps you could introduce a previous prototype of Lamorak as a recurring NPC? An 'older brother'- older, and wholly devoted to Nic and his ideology. Alternatively, have Borne be Lamorak's 'little brother', a later invention.
Not sure what general Book 5 tips to give right now, I'll keep thinking.
