If (and I grant that this is an 'if') each book is a specific series, like the original 4 Typhon Pact books, we know there is DS9 and Titan, and TNG and Aventine seem the obvious two other choices, what could the other book be?
Another TNG or DS9 book?
A New Frontier book?
My best guesses ‘if’ it happens the 5th book would star either the USS Robinson (Sisko’s new command) or a political-based novel with President Bacco and the other leaders of the Khitomer & Typhon Pact members. But it would be good to see the return of the characters from the defunct New Frontier & SCE series.
I have liked the creation of the Typhon Pact so I hope this doesn’t result it disbanding. However with what has happened it could end up dividing into two separate factions – a non-adversary pact consisting of the Romulans & the Gorn and an adversary pact consisting of the Tholians & the Tzenkethi. It would be interesting to see which way the Breen & the Kinshaya go. I don’t know much about the Kinshaya but it sounds like they would support the non-adversary faction. It would be interesting to see which way the Breen would go as originally I could see them supporting the adversary faction but with the changes at the end of Silent Weapons they could easily now end of supporting the Romulans, seeing them as giving the Breen a superior ‘Military’ advantage.
It would be also interesting to see if any of the other non-aligned states got involved. The Andorians are pretty much still divided in both their anti-federation status and their recent actions against the Federation so could easily end up aligned with either faction or even end up aligning with the Khitomer allies (and possibly re-joining the Federation) due to the actions of individual Typhon states. I am not sure what happened but I thought the Talarians were originally going to be aligned with the Khitomer allies but it looks like this is not happening so it would be interesting to see if they end up aligned with either Typhon Pact faction. The Orions would have no love for the Typhon Pact after Silent Weapons, however with influence of the Syndicate could easily end up with either Typhon faction. Don’t know if their joining the Federation is considered valid, but the Thallonians could also end up aligned with the Typhon Pact given the ‘attack’ by the Federation in Blind Man’s Bluff. Also the Reman’s have history with the Romulans, Federation & the Klingons so it would be interesting to see who they would end up supporting.