Lets look back to Atlantis. It was the predecessor of NinjaGo and in january 2011 it did get a small, final, wave of five sets. In the same release Ninjago stormed into the lego universe. I don't remember with how many sets but it was well more than the five Atlantis got. And before Atlantis there were Power Miners and it too ended with a small wave simultanious with the big release of Atlantis. So the course of action has been for a while that the big action theme from the previous year is let go with a small release at the same time as the new one gets a big launch with tons of advertisments.
Looking at that the rumors do make perfect sense. Legends of Chima (hereafter: LoC) is most likely the theme replacing NinjaGo. Rumors say that NinjaGo will get around 6 sets and no spinners. This fits with the previous theme getting a small release. The no spinners rumor fits with the rumor of there being a game part (Speedorz according to some claim) to LoC just like NinjaGo had Spinjitzu. Lego want to shift the kids attention from NinjaGo and Spinjitzu to LoC and Speedorz and new spinners would delay that shift.
I have a hard time seeing TMNT as the replacement for NinjaGo. It's a License theme and even though it has similar foundations in the story (NinjaGo might actually have been inspired by TMNT) I still think the storielines and characters are different enough to not cause any confusion. Even though they're ninjas the heroes are turtles and the ninjas are actually found amongst the bad guys. Hadn't there been non-human characters on the bad guy side too it would almost be NinjaGo in reverse... So no TMNT is not the NinjaGo replacement.
There's also the new "Galaxy Quest" rumor and it sure fits in there presuming that Speedorz is the Spinjitzu equivalent for LoC.
Finally on the rumor list there's Polar Xxpress. In the Brickset posting there were said to be no polar theme. But the Lego Rep presentation that is done now is only for first half year (january - may) and there's an Action theme slot in summer/fall (depending on country) whitch might be this polar theme. So here's my prediction on how the release schedule for the rumored sets will look like this: