The campaign will start in the Last Bastion of Atlantis, the city of Katagia. It is a varied city, filled with rival noble families, a strong sentiment for both Atlantean pride and Thulean integration, and renowned gladiatorial battles. No one needs to be tied to these threads, though — Katagia is diverse enough, and in the vicinity of enough other threats, that any adventurer could find a reason to have made their way to what may be the last jewel of Atlantis.
And then we'll take it from there.
In general, anything published by Pelgrane Press, or by Sasquatch Studios for Primeval Thule, is fair game. Other stuff from forums, Vault of the 13th Age, etc. is probably cool too, but you should ask me first.