I’m guessing the announcement of the Han Solo movie title and a Last Jedi teaser.
I’m sure it won’t be a TLJ trailer, because that has its own panel the next day, and also none of the new actors or directors associated with the ST are on the panel. Likewise, no one with anything to do with the Han Solo film is listed for the 40th anniversary panel, and them showing up seems like a pretty random surprise. My money is still on big shiny box set of a new SE transfer that includes the theatrical cuts as “bonus content”, just like the GOUT but hopefully not terrible.