WARNING: The following article contains spoilers for Green Lantern #7, available now from DC Comics.
To say the Green Lantern Corps has been having a difficult time would be an understatement. In the wake of the Central Battery exploding, almost all of the Green Lanterns have lost their powers. This includes John Stewart. But it seems that the universe had another destiny in mind for him. In Green Lantern #7 by Geoffrey Thorne, Tom Raney, Marco Santucci, Andy Owens, Mike Atiyeh and Rob Leigh after experiencing memories of the ancient past, John began to transform, achieving a new form similar to his Green Lantern uniform, but implied to be much more.
His appearance, coupled with the timing of recent events, makes it possible that John has bonded with Ion, the sentient embodiment of willpower that powers the Green Lantern Corps.
Ion first debuted in Infinite Crisis Special: Rann-Thanagar War #1 by Judd Winick, Dale Eaglesham, Geoff Johns and Ethan Van Sciver. It is the very embodiment of willpower that fuels the central battery, as well as all the power rings that come from it. Ion is one of several entities, each embodying the different colors of the emotional spectrum. It originally existed to keep the fear entity Parallax in check, but after the Central Battery was destroyed both of them were set free.
At that point, it was revealed that Ion was a symbiotic entity that chose Kyle Rayner to be its new host. As a result, Kyle was imbued with near godlike power, not only being able to recharge the central battery, but also create entirely new Guardians through his will alone. Ion is not invincible, however. It is possible for its host to be captured and have Ion forcibly extracted from them, such as the Sinestro Corps did to Kyle at one point. However, if this is Ion, then John may have been handed the key to revitalizing the corps.
The manifestation of the power looks incredibly similar to how Kyle did when he was bonded with Ion. But while the powers that come with it have yet to be revealed, it is the timing of these changes in John Stewart that that lend the greatest evidence that this is Ion choosing him as his host. The entity only appeared when the central battery was destroyed, when the Corps was at its greatest moment of weakness. The same thing is happening now. The central battery is heavily damaged if not destroyed and the Green Lanterns are powerless. Ion may have chosen John because in him he saw someone who could save the Corps before it's too late.
Of course, it is still possible that this is not Ion. John was traveling with New Gods from New Genesis. One of them, Lonar, was responsible for John's visit to the past where the change began to occur. He seemed to imply prior knowledge that this would happen. So there exists the possibility that the change wrought in John came at the hands of the New Gods. There is even precedence for it. Hal Jordan briefly became the God of Light during the "Darkseid War" Perhaps John was enhanced by his New Genesan allies. It could be that this power was dormant within John all along, either brought about by his exposure to the Green Lantern rings or suppressed by it. But without the ring, maybe now was the time for John to become something more than he was before.