My notion is to set it 100 years after the end of the series (so in 100 AV), just after the death of Gaius Tavarus Magnus (note: he lived so long b/c of the chala bond). The concept for the setting is that with Tavi's death comes a new time of uncertainty for Alera, since he was the glue holding the tentative alliance with the Canim, Marat, and Icement together. In the mean time, new enemies and new challenges have appeared, with the looming threat of the Canean Vord having passed mostly into myth over the intervening years. This sets him in a position something like his father, who also threw Alera into a time of uncertainty upon his death.

Anyway, here are some ideas for the intervening history...

* Tavi appoints new High Lords to manage the cities of Alera as soon as possible, establishing new Dynasties where necessary that are mostly composed of the heroes of the Vord War. Bernard and Amara become High Lord/Lady of Riva, while High Lord Riva is moved to Aquitaine, Crassus takes on Phrygia while (somebody else - I don't really know who) takes up Rhodes. Veradis assumes her duty as High Lady of Ceres, and eventually marries Max who becomes High Lord Ceres. Forcia remains without a High Lord because it was completely devastated by the Vord, so much so that nothing but ruins remain where it once stood. Attica becomes the military encampment of the combined Aleran/Canim/Marat army while they drive the Vord back, but grows much smaller after the primary thrust of the reconquest is finished.
* The Vord Queen, just prior to her death, gave the order for a new queen to be let free of her cocoon in Kalare. This new queen was altered so that her instinctive drive would not be to conquer, but simply to survive. The aim of this is to allow the Vord to keep a beach-head in Alera so that when the Canean Queen(s) come back they will have an easier time invading. This Queen then goes underground, turning the Vord that go with her into cave-dwelling monsters who strike from below and consume any steadholders or armies that venture into their territory.
* The Canim and the Alerans, combined with the forces of the Marat, drive the Vord back to Kalare within a generation. The croach is destroyed and much of Alera is freed, though the area around Kalare remains wild and not under the control of any organized power.
* Having mostly defeated the Vord, the Canim then return to their aggressive ways and fight to take Attica as well as Parcia. They then allow the Free Aleran and others of a similar vein to live and work there, establishing it domestically as a Free City even while imposing military rule and turning the city's products and profits toward their engines of war.
* A lone Marat, outcast whelp of Herdbane clan, comes to form a Chala bond with a Vord through a series of unusual events. She is then the progenitor of Vord Clan, a small but growing clan of Marat who are by degrees tricked, forced, or coerced into forming bonds with Vord. These Marat turn to a sub-surface life in the Vord-dug caves beneath Kalare (and mount Kalus in particular), providing a new and dangerous foe to those who seek to retake that wild land.
* The Children of the Sun, long thought extinct, return from over the sea (in Maratea) to their ancient home in the Feverthorn Jungle. They take back what they once held, and though they don't take much more than that they make that country unfriendly to outsiders.
* Just a few months before Tavi's death, the Royal Navy in Westmiston is attacked and dealt a rather mighty blow by what appear to be a significant number of Taken Leviathans. Riding on their backs and hiding in their stomachs are Canean Vord - an expeditionary force from the gigantic Vord Nation in Canea.