Erikstormtrooper said:

What was the purpose of keeping those characters a secret anyway? There were so many other aliens we saw beforehand; why hide the Ewoks? It sounds like something George wanted for one of his "reasons".

If I remember audience reaction correctly, seeing Jabba in all his sleazy glory for the very first time was worth the secrecy. A movie should have a few surprises left up it's sleeve when you see it for the first time. I don't think we saw anything of the Emperor's appearance in Jedi beforehand either.

I'd wager the teaser is going to stick to familiar characters and things we already know about.