The surprise they keep talking about at the celebration could turn out to be nothing more than the Ep. 8 trailer.
The opening panel of the 2015 celebration was specifically about TFA. They even said ahead of time that they’d be debuting that trailer at that panel IIRC.
If one of these “surprises” at a panel about the 40th anniversary of the original film was the TLJ trailer, we would know it.
ETA: CHEWBAKAspelledwrong beat me to it