If you haven't already left from the spoiler warning in the teaser, I am going to warn you again, don't keep reading and then complain about spoilers.
I myself, am a huge X-Men fan, and believe it or believe it, I am also an advocate of the Fox X-Men films. I feel that most of the films they have done with franchise have been great. That being said, they handled two of the greatest X-Men stories of all time horribly (Weapon-X/Origins and The Dark Phoenix Saga). So I wouldn't be all too surprised to discover they butchered yet another great story and villain. That being said, I was still surprised by what seemed to be how they were depicting the second greatest X-Men villain of all time, which was all too reminiscent of Smallville's depiction of Darksied.
Let me say this, Fox didn't win my heart back easily, even after First Class we were still weren't on steady ground, but I felt after The Wolverine, they knew what they were doing. So if this scene is to be taken at 100% face value without context, I am not okay with it, and Fox has lost me again. But I don't feel we are looking at this scene in context. So let's do that.
This X-Men world is seemingly one without real time travel, in DOFP, we will see Wolverine's consciousness be transferred to his younger body, not actually moving physically through time. In the scene he apparently comes from the sky, after "crossing the rivers of time," meets Magneto, and merges with him. I would like to first note that the original article from Firstshowing.net stated
"an existing X-Men character will become possessed by the villain who can project his consciousness and powers into host bodies."
Now let's remember that Apocalypse is a villain dealing in the future and alternate futures quite a lot. Lets also note that some old X-Men cast are set to return for X-Men: Apocalypse, and Fassbender is seemingly locked in. So it appears we will be having a split between past and future again. So if the only the only way to travel back in time is to go back through your past consciousness, and the only person who can do this is an older Xavier, and both timelines will be used AND Apocalypse is mainly a future timeline villain, I think the rest becomes fairly obvious.
At some point Apocalypse, the real apocalypse, of the future, will find Xavier and take his powers away from him. Being older, wiser and all around more powerful than Xavier, he might just find away to transfer his mind and powers, to not just his past self, but anyone's past self. Maybe he is able to appear to those he will control via Astral projection. Perhaps he will use Magneto to cause chaos in the past and have it been seen as "typical Magneto," to change the future to suit his needs. And maybe even while under Apocalypse control we can see a more comic like Magneto, with his eyes glowing white and his face concealed in darkness behind his helmet
And if I may grasp at even more straws, you may even say that Charles used this astral projection time travel power in the past, to guide a young Scott Summers off of long mile island, and to the past Charles Xavier, given that Scott escapes in the 70s and DOFP take place in the 70s.