While I do love the idea of Daravon, that will likely have to wait for an "Xtreme Legends" pack, heh.
Agrias' story will likely revolve around her teacher, Katelyn.
Lavian and Alicia are there only to hone their skills, I would think that Ovelia would sneak out of Orbonne, not wanting to be left alone, and convinces Agrias to take her into the group when she lets her cover get blown before reaching Dorter.
As for Cameos... FFXII RW introduced the Cache of Glabados which brought Balthier to Ivalice. As for Vagrant Story... that takes place 100+ years after FFT, but... if the Cache of Glabados exists in the past, it likely exists in the future, the only one I could see though is Ashley (Vagrant Story has a pretty large body count, where the plot is concerned...)
Cameos are a tricky balancing act though.