Dear mother of god...I don't post here often, and while Ninjago has never really been my favorite theme, the fact that there is a possibility of it being replaced by TMNT is horrifying to me. At least when I see Ninjago I know that it is a deep storyline made with TLG's own blood, sweat, and tears in a world where almost half of its themes are licensed. If we accept TLG's new corporate strategy of conflict and storylines then in my opinion Ninjago epitomizes how to do it right (not to mention it had some well-executed sets for what they were).
Aside from ranting and onto constructive discussion- indeed Legends of Chima sounds the most likely, likely because it's similar to Ninjago and could possibly ride off of its success. Moreover, if it indeed has an Oriental theme to it (pretty arbitrary, my second best guess is South-American) then maybe they could be encouraging kiddies to integrate the Ninjago and Chima sets? I remembered TLG combined the canons for Atlantis, Alien Conquest, and Pharoah's Quest so there is precedent.
EDIT: The TMNT article that mentioned a "reliable source" said a release date of January 2013. So if it is to be believed, and since we know we have one more wave of Ninjago, then we know it's not "replacing" the theme.