I think Alison is a fine talent and Robert Plant made a wonderful choice in her regard. He'll probably entertain each possible re-encounter with Zeppelin on a per instance basis. It will depend on how he thinks about it at the time, and if he feels inclined and inspired. Even when Zeppelin was going full force, Sandy Denny made several appearances, and was quite lovely. We may hear a new rendition of the Battle of Evermore yet.