I suspect they will have some footage to show, they already have alot of the cgi work from the previous move, so it's not like they are starting from scratch.
It will not be very long, just enogh to wet everyones appitite for next year. They have to be at this years comic-con due to the fact it's the only one before next years release. So they will have some stuff, but I can't imagine they will have anything that gives the story away or gives away any surprises.