The most commonly cited theory for Snoke is that he's in charge of the entirety of the First Order. This theory says that since the leader of the Republic was the Supreme Chancellor, it makes a kind of sense for the First Order to be headed up by a Supreme Leader. There's a symmetry there, if nothing else. The humanoid figure glimpsed leading the rally in the trailer is almost certainly not going to be Supreme Leader Snoke, because that guy is plainly a human. However, just becuase Snoke isn't shown at all in the trailer doesn't mean that he can't step into view later in the rally scene, in all his non-human splendour. There's one problem with this theory, though. If the First Order is as racist as the Empire was, then having a leader who's a filthy alien might be a bit hard for the troopers to swallow.