You know, the way I would want it done, if you go with the best possible ending(IMO only), Yen and Geralt preparing to leave for their new home with Ciri as Geralt's witcher student in tow. They have to go out first to clear out the old estate that Yen has picked out...she would know about it from her travels. Would seems simple enough until Geralt and Citi discovers the estate sits near the ruins of an old library, one of the first places Avallac'h had researched at when he first arrived here!
So, why would Yen be interested in the sage's studies? What would she be hiding and what is her motivation...clearly it would have to be pretty serious stuff, LVL 40+ creatures and special gear drops! They would discover Yen might be trying to learn more about Ciri's gift and following the sage's early steps...it would seem nefarious to Geralt but in the end she would finally share that she is just looking out for Ciri, trying to shield her from another elven sage that has come onto the scene. They would enlist the service of several old friends(step right up and spin the wheel!) and in the middle of the expansion the two witchers would have to split up! That would give us a chance to play Ciri on a full quest solo!
For the fun of it, a need for Geralt and Yen to visit and consult with Triss about something could arise...this would give the fans of that sort of thing a chance to see the claws come out a bit...I won't think of trying to guess how that part of the story would go but, it would offer some interesting make-up-time for Yen and Geralt upon their return home to met up with Ciri...and all of this stuff would offer a lot of conversations along the way.
In the end, the 'family' would have to combine their strength to overcome the new sage and beat down his minions...for the win.
When its all done you would have everyone that survived in the main story where they would be a year later. The would be static, limited interaction but they would be in the world with the player...in the POST war world...and it would feel more populated and less empty when know where everyone is.
Besides, that would give all kinds of jumping off points for future DLC as well fan-made content creation. "Go talk with Triss and get an item..." type stuff
Does any of that make any sense? I am on meds right now and still kinda emotional from finishing the game last night late