Serious speculation only please, I don't want to lock this thread due to spam.
I'm hoping the Preview will be 30 seconds like the updated 2.0 one was, not 15 seconds like the updated 3.0 counterpart. As for the content...awhile ago, TM suggested maybe it'd be one scene (similar to Asuka in space for 15 seconds) of 3.0 Rei listening to Shinji's Sdat. I could actually see that happening.
Though what I'm personally hoping for is semi-finalized (animation's a process) shots of Shinji, Asuka and Rei in the red desert storyboards from Peaceful Times. Or, it could be a proper (updated) version of the Eva 02 + 08 battle, similar to how 2.0's Preview was revised contents of the original Preview, updated with in-film footage. Of course, 3.0 was a different story (comparisons like other link).
I have a hunch they'll avoid showing any shots of Nerv/Seele (Kaworu returning, whatever) or Wille members (to leave things up in the air about their current plans and/or reactions to Shinji), but man oh man, if they end up showing a shot of post timeskip Kaji or the now adult classmates, I think everybody will flip.
