I’d love a series about the fall of Ben Solo, it’d definitely be more dramatic than a book about it.
As for the Celebration surprise(s), someone mentioned earlier the possibility of it being a Lucas appearance. That would be fitting, and I don’t think anyone said the surprise would be any kind of announcement. If anything, the big surprise being merchandise of any kind, including a new home media release, would probably get an underwhelming “making the fans buy the same movies again” response, OOT or not. That doesn’t sound like a fitting enough surprise for such a huge panel.
It will probably be Ford and Lucas. But neither of those is really a big deal. Short of the OOT or Carrie back from the dead, there’s very little they can do to “surprise” us.