Halo Wars was more than just a spin off, it was maybe the first RTS that just 'worked' on a console- although it didn't hurt to have that Halo branding on it. But with little to no chatter since the title was released and the disbandment of the game's architects, Ensemble, it didn't seem like a sequel was something we should look forward to.
But maybe that needs to be rethought. 343 Industries' Frank O'Connor talked with CVG recently and called the game "a huge success", citing better than forecast sales numbers and critical acclaim. He also said that the thought of a continuation for the series was not out of the question. Before you get too excited, lets add on there that O'Connor also said that there's nothing being worked at the moment.
One very interesting thought? O'Connor mentioned utilizing Kinect. Gesture based Real Time Strategy? Now that has serious potential.