I think the challenge they face is that they built a strong story line with several great individual character movies, combined them all with the Avengers, and did an excellent job, then ended the story of many of the most beloved characters they had (which were also some of their strongest actors too).

There is no more Steve Rogers, Black Widow, Iron Man, Thor, or T'Challa to look forward to. And it's not clear if we'll ever hear from some of the other Avengers characters again, not that many of the remaining Avengers could carry the story on their own without the retired characters/actors. Marvel will be hard pressed to get new characters and story lines that match what they did with Avengers.

Can't help but think that not having Stan around is really starting to hurt them too. I don't know how involved he was with the Avengers story, but that could be part of this.