I've not heard anything about Merida, beyond hopeful speculation. It would be awesome if she got a figure, but I think we would have heard at least whispers about her by now if she was being released this year. We'll have to see.
I think the girl pack is something being released for the Christmas market, especially to appeal to girls, so it'll be a combination of already released figures. I'd wager it'll have Rapunzel, Anna, and another female character, most likely Vanellope, rather than anyone completely new. It doesn't make sense to put both Anna and Elsa in it when there's already a pack of them both. Also, I don't know that it'll be good value in the way the sidekick and villain packs are, since you might want to buy Vanellope and Anna in the toy box packs and get the potential extra toy box content from them. I think it's just a Christmas thing, something attractive to parents looking to buy for their kids who may not scour the web for Infinity info like we do.
That said, if there is someone new in there, I imagine she'll also get a separate release. Disney are trying quite hard to fight the accusations of tricking parents out of money that have been thrown their way over the last few months, so having a new figure only available in a pack with two other figures people might already own, forcing people to buy doubles, would fly in the face of that intent. But I just don't know how likely it is we'll see a completely new character.
Of course, I could be completely wrong. There could be a new character. It could be Merida. It could be three new characters. It would probably be the happiest I've ever been to be wrong!