You shouldn't really expect anything, going off of past Treehouse events.
There almost HAS to be a trailer at ONE of the events though. Sakurai made it sound like they had at least 2 (maybe) reveal trailers after Robin/Lucina, meaning Shulk and one more. If there IS one, they HAVE to reveal it BEFORE the 3DS game comes out, otherwise it defeats the purpose. Nobody is going to be excited for a reveal trailer for a character that they already have access to.
If NOA/Sakurai were smart, they'd attempt to cut off leakers/Japan and just have a massive reveal of the full cast and have it be kind of similar to the SSB4 reveal trailer, showing off tons of characters, or multi-reveal like Robin's or Greninja's.
There's no way they can stop early leaks, but NOA will have lost any excitement they could provide once the game hits Japan, which is the only reason they're going to stream the build in the first place, to direct attention to them, so not revealing anything would be dumb.
I don't care if they reveal everything, or more likely don't, but it will be a missed opportunity to show us high quality footage/trailers of all the characters being our first view of them instead of more lulzy quality pics & video.