Now that I think about it, actually, there are some interesting differences between what we've gotten from castle over the years. In the beginning there were semblances of story, but nothing concrete; more open for kids to make their own stories. There were a bunch of jumbled themes, from the Bat Lord and his knights to the Lion and Bull knights and forestmen. Then somewhere in there Knight's Kingdom came along with a full-blown story behind it. After that came a slew of Castle themes, each bringing neat factions and such, but no solid storyline. Even the jewel of the castle themes (IMO), the fantasy theme we got with the Crown knights, dwarves, skeletons, and trolls, didn't really have a story behind it.
So that begs the question: if Lego were to bring back castle, would it end up being a story theme or another one-or-two wave, storyless theme like the ones we received after the fantasy castle theme?
If it were handled properly, I suppose it would be pretty neat to get a full-blown fantasy castle, story-driven theme.