After viewing the trailer a few times, I have pieced together a theory for what I think the plot of The Lego Movie 2 sill revolve around.
Right from the get-go of the trailer, Wildstyle says that a lifetime has passed since the original movie. This implies that people have grown up, including the young boy from the first film. Now that he's older, my guess is he doesn't play with his/his father's Lego bricks that often. That's why all the minifigures and the like are in such a desolate wasteland; they're unused and must survive on their own.
Now, within this passing time, the boy's younger sister has also grown older and now she plays with more age-appropriate sets for her (hence, the mini-doll figure), rather than Duplo. My guess is that she is taking her brother's toys and adding them to her own collection (Emmett's friends being captured by the mystery mini-doll and being transported to a more pink and white colored setting).
In short, I think that The Lego Movie 2: The Second Part, will revole around the concept of growing up, and passing what you love to the next generation.
What are your thoughts about the plot of The Lego Movie 2?