An unknown would be nice.
Leave her out altogether? I can't see them doing that ONLY because Finnick's so important. He has scores of admirers he's benevolently (maybe?) tolerant of. They need to show someone he's not blas'e about. Finnick's tortured over what's being done to Annie. He can't hold it together like Katniss.
The wedding is one of the few bright spots of Mockingjay, as well as Katniss being able to see Peeta is in touch with his painter/cake frosting craft. That would be interpreted that he might be getting better, remembering enough to do such an exquisite cake.
BUT, playing devil's advocate, they left some things out of the first book, and it didn't hurt the movie at all, IMO. I guess they could leave Annie as just a flashback. But then we wouldn't get to see Mags' heroic substitution. There's no way Mags thought she was getting out alive.
I want Kristen Bell. A redheaded Kristen Bell. But there's no reason to think she's in consideration. Annie's mostly got to act pathetically unhinged, until she reunites with Finnick.
They're already at work on "Catching Fire". WoooooooHOOOOOOOOOOO!