It could be that the after-credits scene of Avengers: Age of Ultron is where we'll learn which characters will join the ever-changing line-up. And one long-awaited name has always been on the list.
Spoiler warning here, although it's not really plot-related.
According to Badass Digest, the end of Avengers: Age of Ultron will have Captain America revealing a new team, made up of both familiar and new faces. The scene will reportedly take place far enough from the main action of Age of Ultron for the characters used to have acquired their powers/costumes/etc. And Badass Digest reports that the members are still up in the air until they actually film the scene. This seems uncharacteristic for Marvel's usually years-ahead planning process, so I'm taking this all with the requisite grains of salt.
What's most interesting in the article is that the scripted version of the scene, written by Joss Whedon, has always included Captain Marvel. But even that may not be enough to guarantee that Captain Marvel (Ms. Marvel in the script) will actually end up being seen in Avengers: Age of Ultron. Even though Devin Faraci says that his sources say she's definitely coming, he also says that the Marvel execs aren't too keen on dropping her into this scene, fully formed. The same reasoning apparently also explains why Black Panther is missing from the film, even though Ultron's looking for vibranium in Wakanda.
So, if she's not showing up in Age of Ultron, does that mean we won't see her until Marvel can find space for her in her own movie? Or introduce her as a major character in another movie? Or have her history be a major part of Avengers 3?
There's also the possibility that this scene hasn't been shot and Marvel will get over their cold feet before it is. Plus, if this is true and we'll be seeing the future Avengers in the after-credits tag, which new characters are Marvel comfortable debuting? If not Black Panther and Ms. Marvel, then who? Will it be the first glimpse of Ant-Man? Or something we haven't guessed so far? Mostly, I hope wishing for T'Challa and Carol Danvers will be enough to make it so. Fill in the blanks later, Marvel, and just show us a commitment to these characters ASAP.