The problem is that a lot of classic Lego lines haven't survived; things like the last castle line, pirates line, agents line, etc.
Thus I think they'll probably try and do something pretty new, although my personal hope is that they can make a fantasy castle line with a true story; I feel that the story is essential when it comes to maintaining a line. This is one of the reasons why Bionicle G2 failed; the story was super simplified and not readily available to kids. This is why Ninjago sells so well; kids don't want the sets for the vehicles themselves, they want the vehicle because it is Jay's vehicle, or Kai's vehicle, or Zane's vehicle, etc. and it contains their favorite character.
I think part of the draw to the story as well is the theme slowly maturing; think about Bionicle G1; it started out rather simple, but as time went on it got a little darker, to the point where a main character died! It's fine to start off simple, but if it doesn't get any deeper than that your group is going to age out and the new kids won't be as into it since they didn't start with it.