【特報】『魔法少女まどか☆マギカ(まどマギ)』の続編を構想中であることをアメリカのANIME EXPOで虚淵玄氏が明らかに!「今シャフトと一緒に、必死に続きを考えている最中。」
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「魔法少女まどか☆マギカは、勢いに任せて書ききった作品。当初はまさかこれほど世界中の人々に見られるようなアニメになるとは思っていなかったと述べ、続編については、今、必死にシャフトと一緒に続きを考えている最中。」
Well later interviews and such have claimed that Urobuchi only initially imagined the ending the way you mentioned, but he himself had doubts as to whether that would really be in character for Homura and so they started discussing other options... Then again, and this might just be my cynical side who still preferred the show's ending (though to be fair, I am a lot more okay with the movie now than I once was... I mean, I did end up buying the BluRay), it's entirely possible that that's just him doing typical PR-speech to get more money because even if he did dislike the ending he would never come out and say that.^The reason why the ending became like this was precisely because Shinbou wanted to leave the possibility for a sequel open so it's not unexpected.
The original was a good end, by the way. Urobuchi had it written to be the end, where Homura leaves quietly with Madoka. Shinbou wanted it changed. And so, here we are.
Opens with Madoka and Homura giving an overlapping monologue. The whole thing turns into a ballet dance. We see Sayaka change into a magical girl and then into a witch, we see Kyouko reach out for her but miss.
We see Mami dance with Nagisa, and then there's a tempo increase where Homura and Madoka start swerving closer and closer to each other, then Mami swoops in towards Homura who swoops back.
It ends with the ballet being broken, and Homura and Madoka in a isolated field, backs turned towards each other
Homura's actions backfire in the worst possible way and her being faced with a choice where she can only partially fix things