In an earlier article on the allied-color minor gods of Theros block, I mentioned that although we more or less know whom the red-green god will be, that character's story is too complex to be just a subsection of an article. The Magic rumor community has been able to piece together some big hints, including accidentally-released Hero's Path images from Born of the Gods and Journey into Nyx, and determine that the red-green god is a known character in those colors - Xenagos, the Reveler.
According to Jenna Helland's biography of Xenagos at the PAX Prime worldbuilding panel for Theros, seeing the vastness of the multiverse made the wild Satyr feel like a speck of dust, so instead of trying to be a proverbial small fish in a great big ocean, attempting to take over a single plane suits his megalomania better.
Near the end of the Theros trailer, after Heliod is done with his exposition, he says (to Elspeth) that there is a great threat facing his plane that she must overcome - and the silhouette of a four-horned face briefly flashes on the screen. Those horns caused the image to briefly be mistaken for that of Nicol Bolas, but once placed side-by-side with Xenagos's picture, it becomes clear - Heliod is foreshadowing Xenagos becoming a main villain.
So what's the next step in his plan? The Born of the Gods Hero's Path images definitely show a gigantic projection of Xenagos complete with a Nyx starfield projecting from his body - there's no question he manages to reach godhood.
Then, the Journey into Nyx picture has him trying to overwhelm Elspeth and Ajani in a vicious battle. Significantly, the Planeswalker Points achievements for that set are listed as WIELDER OF A GODSLAYER, NYX EXPLORER, and DEFEATED A GOD - perhaps the Game Day challenge deck for the Journey into Nyx stage of the Hero's Path pits players against Xenagos himself?
This all begs the questions - how does Heliod know what Xenagos is trying to do, and why can he not stop him himself? After all, he's one of the five most powerful gods of Theros, and could easily rally most of his fellows against Xenagos - a newly-ascended minor god would be, to him, like a minnow to a whale. The most intriguing line of speculation involves the marked resemblance between Heliod and Xenagos in face, horns, and skin tone - which could even be said to be familial. Greek mythology is full of patricide, and not only did Kronos kill his father Ouranos in the Theogony, Kronos's son Zeus overthrew him to be king of the gods. Simply put, Heliod may not be able to bear the idea of battling his own son.