Teridax going inside what seems to be Mata Nui's BRAIN for his FINAL TASK might add to the possession theory.
I'm also a fan of the Brotherhood-gets-killed theory. That is, Teridax killing the Brotherhood off. I mean the Makuta in Karda Nui are most likely screwed, let's face it. And I'm pretty sure it was said that Teridax predicted the existence of something like the Order Of Mata Nui and factored it into his Plan. Which means, he would have factored the resulting battle into his Plan. And consider, he sent his best fighters to Karda Nui, leaving the rest to deal with the war.
I think this theory might hinge on the entire Brotherhood consisting of around twenty members. And Greg has always avoided answering how many there are, so this may be it.
Translation -- I think Teridax has planned the death of the Brotherhood.
I know this's been theorised, but this one is "NEW! Now with more evidence!"
Hey wait, this is the 2009 thread.