Feel free to write an epilogue! Knowing that you guys finish this up as level 18 saviours of all of Avistan if not most of the planet.
Hmm... I'm not sure. I've never really thought about it.
Thaddeus would certainly seek to learn more, staying a while with the Cyphermages to record everything that's happened (And give a full analysis of the Drow. To hell with the winter council.)
Given conversations with Fiona, he'd ask her what her long term plans are, if she seeks to return to the Shoanti. If so, Thaddeus may ask to go with her and record everything he can about the people and the tribes. Their traditions, oral history, and so fourth.