Currently Airing Anime

 speculation stuffs

Opening might possibly be showing a different timeline where Madoka actually does become a magic girl. If you notice, Homura doesn't have many appearances in it. It's mostly Mami, Sayaka, and Madoka. Homura may not have played too much of a big role in the previous timelines, and she went back to prevent Madoka from becoming a magical girl, for whatever reason. The OP shows the three having a nice time, with fun magic girl adventures, but that's obviously not what happens. It also shows some times where Madoka is crying, or running somewhere far, while staggering and such.

 Spoiler

Ooh, you do have a point. Mami was depicted as 'alive' during the opening. I'm going to completely make a guess on this, but I think Homura was probably the last Magical Girl in an entirely different timeline and must have met Madoka way before. Something must have happened to Mami, Sayaka, and Madoka in the other timeline and I'm guessing that Homura is going back in time to stop them. It actually does make sense, because you're right about how Homura wasn't shown a lot in the opening. I'm just rehashing what you and several others said, but yeah.

Or, in the case that I am wrong, my 2nd speculation will be that Madoka somehow will make a "wish" that prevented all of this from happening. But it's going to be some sort of wish that should do two things:

a) Mami didn't die
b) Sayaka is still a Magical Girl

I guess it's hard to say at this point, because we're over half-way and there's still not a lot of hints as to what the outcome might be. They have to make a 2nd series though, assuming this will be a 12 episodes series. Otherwise, I'd feel as if there's a massive hole in the story that a lot of people won't understand.

Also, I always wondered about this. Assuming that a person was about to become a Magical Girl, couldn't that person make a wish for herself to grant invincibility / invulnerability? That way, the person would technically never die or get damaged. Or what about immortality? I'm surprised this hasn't been brought up in the series yet. INB4 indestructible Magical Girl. `-`But then if the soul gem would be too far from the body, I guess they would die regardless. Hmm.