What they need to do in the next movie is guest star one or more heroes/characters (if only in the background). Someone like Daredevil (only show him in costume and recast the role; Affleck was not a good choice), Black Cat, Dr. Strange, etc. Imagine Spiderman being saved or helped by another hero at the last minute. They have a few words of introduction and thanks…
If they do it well, they can lay the ground work for future movies/characters to a wider public that does not necessarily follow the comic cannon.