This is, IMO, the best answer in the thread. The LEGO games work best when they're mixed with a well-established franchise with a lot of material to mine. This is why the three franchises already in use worked so well.
Jurassic Park Trilogy
If the rumors of Harry Potter Lego games come to fruition then I will be happy as well. Lego Superman or Justice League could work quite well although I feel a Marvel themed Lego game would fall flat.
I can't think of any other franchises I'd like to see rendered in block form. However, now that the Lego games have become a juggernaut on their own I would like to see original adventures based in the Lego brand universes (i.e. City, Castle, Space, but NO Bionicle) and/or the Lego MMO brought to consoles. I'm not looking for much in the way of story here. I just want an easy game I can breeze through mindlessly and rack up the trophies/achievements while I'm at it.