SPOILERS- IGNORE POST IF NOT FINISHED THE GAME!!!
for the free dlc:
wolf school gear treasure hunt quest
alternate look Triss
alternate look Ciri
more side quests involving fellow witchers/beef up kaer morhen with more side quests
more qwent tournaments, horse racing and fist fighting.
storage area for your items (though this should be really a game update and not a dlc)
more content with Triss romanced or not
Expansions
Post ending DLC involving Ciri perhaps set in Nilfgaard, characters to reappear- Geralt, Ciri, Yennefer, Triss, Dandelion, Zoltan, Emhyr, Avallac'h, Phillipa etc.
the plot could change depending on the endings for example:
witcher ending- emhyr pissed off that geralt lied to him about ciri and wants revenge.
empress ending- ciri is nearly assassinated, geralts job to find out who and butcher them.
ciri disappear ending- geralt dreams of ciri causing him to track down Avallac'h (who is in Nilfgaard for some reason) for answers, but the elf is scheming up something.
would be hard work to make due to the endings being so different but I believe CDPR has what it takes to make it.