Joss Whedon will reassemble The Avengers one last time in January, as Avengers: Age of Ultron does some reshoots at Pinewood. This isn't a particularly unusual situation, and Marvel Studios always plans in reshoots as part of their budgeting and scheduling. For some filmmakers this is a terrible way to work, but it's the Marvel way, and it'll be the Marvel way for the foreseeable future.
The initial report on these reshoots indicates that the entire cast will return for more action scenes, but I wonder what else might be shot this January. My speculation follows, and includes some minor spoilers for Age of Ultron:
In the film Ultron goes to Wakanda to get his robotic hands on vibranium, the same supermetal in Captain America's shield. The latest version of the story that I know does not include Black Panther in these sequences, but now that we have a T'Challa hired in the form of Chadwick Boseman might the superheroic king make his way into the film? Having Black Panther show up in the post-credits scene, mad that the Avengers came and had a fight in his closed-off nation without even reaching out to him, could be excellent and set up the character for Captain America: Civil War.
As I've reported in the past, iterations of the Age of Ultron script have ended with Captain America assembling a new team, one that appears as basically a cameo. That team, in the versions I've known, contains good old Captain Marvel, who is getting her own movie. With Carol Danvers almost certainly not appearing in Guardians of the Galaxy 2 (James Gunn told fans on his Facebook page that he didn't want Richard Ryder in the sequel because he wanted to keep Star-Lord as the sole Earther in his movie, which means no Danvers), this is a great place to introduce her, even if it's quickly.
I think that may be how Marvel will do it. They're easing us into Black Panther in Civil War, and I would bet that Captain Marvel appears in Age of Ultron and Infinity War Part 1 before her own film comes out. This is a reversal of the Phase One scenario and more of a traditional spin-off scenario. It makes sense at this point in the Marvel long game to do it this way - they made some big splashes right up to Guardians of the Galaxy and now they have plenty of launching pads for new characters.
As far as I know that scene of the new Avengers was not shot during the initial schedule (if I'm wrong let me know, but this is what I've heard). That means it could happen in January (or later - remember, Marvel shot the post-credits scene on The Avengers after the movie premiered), and if so there should be some Captain Marvel casting happening in the coming weeks. Scoopers, start your scooping - I'd love to know who Marvel is meeting with. Of course, as the negotiations for Doctor Strange enter their sixth month maybe the Cinematic House of Ideas is finding it harder to tie people down to these big contracts.