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What's up with the other pink-haired girl in the OP? Are we assuming that's older Madoka?

Obviously that's just Kyubey molesting her to turn her into a magical girl, like he did to Sayaka. Or should I say like she did.

No really, that is a bit of a mystery.

 Spoiler

I was thinking... what's Kyuubey getting out of this? He's not taking their souls, just moving 'em around a bit. He only eats the seeds after they're used to soak up some corruption from the soul gem. Either he's a unique trash disposal (inb4 Kyuubey trash can that opens on his back), or he's getting some sort of evil power from those evil seeds. Or maybe girls sins just taste delicious.

 slight speculation that still seems pretty likely

The grief seeds are tainted soul gems of magic girls. When the gem gets completely black, they probably turn into witches, but it probably takes a lot of sin for it to become that black. Someone took this speculation and made a really nice chart of it last week:

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And Spidey, you are (and/or hang out around) people who are way too good at speculation. No idea how you can have so many ideas about why what's what.

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Well, I read lots of mystery stuff, and those are all about picking up on small clues and trying to find out what's going on. When watching this and talking about it online with others that are doing this as well, we pretty much help each other by picking up on things the others didn't get, or building up on each others ideas. I honestly wouldn't have been able to crack the runes that appear in the series (and no, they haven't been abandoned, someone posted that in EP7's witch fight the runes were still there, but they were kind of blended into the background). Stuff like that can only really be done with cooperation, I don't really think it's something a single person can do quickly.

And of course, lot all the speculations are correct, but when you create a lot of plausible ones, one of them is bound to be at least partially correct.