Really, it doesn't have to be either a repusor or a power source. And the only time glowy circle equaled power source was in the movie. In fact, Stark seems to make a point of making power cells not glowy in the comics. It's a glowy circle on the wrist. The only other time I've seen that on an IM armour was the Crossing armour's heavy duty repulsors, but those were part of a huge gauntlet. It could be a new forcefield projector or something.