There's a mod out in which Ciri and Geralt track down the last Crone and slay it. The modders did an amazing job putting together existing lines from various characters and create new dialogue.
Ciri's Sole Memento
I also noticed this on Nexus. Interesting, and rather ambitious, idea. This story point was always a loose end I rather wished the REDs had allowed players to tie off in other endings. Perhaps I'll give the mod a whirl someday.