Hmm... well there is the Theories board, so most speculation would fit in that category. Speculations on what certain characters might do could fit under Theories, Characters, or Relationships, depending on what the point of view is, while more general series speculation could go under Theories, the Korra gen board for LOK-specific stuff, or the Avatar: Universe gen board for other/more vague series stuff.
.....So yeah, it is spread out all over. ^^; But speculation is such a big part of discussions, it'd be hard to keep it confined to one board. But for overall Korra speculation, yeah, I would say it would go under the Korra General Board (for the more vague speculation/hope-to-see kind of things), and Theories (for the more specific, here's-some-evidence or interesting discussion questions kind of things).
Although, that's just me seeing it right now. I'm sure things will be modified as they need to once this adventure starts rolling again.
Last Edit: Mar 23, 2012 3:14:55 GMT -5 by Musogato