It's common in fanart to include Charlotte with the other magical girls and Kyubey as though she was some kind of extra mascot. A commercial released just before the premiere of the movie seems to have this actually happening.
The film also canonizes some of the ideas about Kyouko and Sayaka's relationship (though it doesn't quite go all the way). Among others, it reveals that Sayaka did have a regret upon passing, of not being able to meet Kyouko on better terms, showing that their relationship wasn't one-sided.
Dueling Movies: Premiered during the same weekend as the non-serial movie of Doki Doki Precure. $hinbo really knows his marketing (see right below). FYI, Rebellion wins.
Executive Meddling: While downplayed compared to other examples, this movie contains an example of this, as Urobuchi planned to have the film be conclusive. Both Shinbo and Studio Shaft wanted to make it more open-ended in case they wanted to do sequels and the suggestion from Shinbo actually helped Gen write the ending in a way that would do just that. Unfortunately, the howdunnit ( Soul Gems are capable of being corrupted by love) is an Ass Pull that contradicts the rules of the series ( despair and usage of magic corrupt soul gems) and came with no foreshadowing that it was even possible. This contradiction has made a lot of fans rather angry.
What Could Have Been: Originally Rebellion was plotted as the anime's second season. However, the creators couldn't find a way to make it long enough. Then some other staffers began wondering if they could make a movie adaptation of the anime...