Marvel likes to tease the next movie with the post credits scene from the current one. They will stick with that formula, as "Captain America: The Winter Soldier" has scenes teasing the new Avengers-The Scarlet Witch and Quicksilver.

The spoilers were taken from Brain Freeze, translated from Belgian via Screenrant:

"Scene 1:

After the fall of both SHIELD and HYDRA we see Baron Von Strucker (Thomas Kretschmann) in his underground laboratory. His men have the scepter of Loki got hold of, as well as two special prisoners.None other than Quicksilver (Aaron Taylor-Johnson) and Scarlet Witch (Elizabeth Olsen). The superfast Quicksilver bounces from one side of his cell to another, while Scarlet Witch few blocks stacks with her thoughts. Von Strucker proclaims that "The Age of Miracles" is near. And thus we immediately know how mutants are listed in the Marvel movie universe.

Scene 2:

We see the Winter Soldier incognito Captain America-exhibit at the Smithsonian Museum. Here he encounters the information about James Buchanan "Bucky" Barnes. It plays threatening music, but this Winter Soldier seems indeed to have who he really is and that Captain America was telling the truth."

With this reveal, we have some spoilers from Captain America itself. Perhaps the most curious revelation is the 'fall of SHIELD and HYDRA' which means that Baron Von Strucker will be working as an independent villain.

It was already revealed that that the Thomas Kretschmann (Baron Von Strucker) has signed a multi-picture deal. That means, the character will stay beyond "Age of Ultron."

As for Captain America, Anthony Mackie (Falcon) has mentioned that it is more of "Avengers 1.5" as Brain Freeze confirms in their review: "Captain America: The Winter Soldier is the most direct sequel to The Avengers and the most direct prequel to The Avengers: Age of Ultron. It's this movie that drives the Marvel movie universe forwards. There's also a lot namedropping going on, from Iron Man to Hulk to even Dr. Strange."  The scepter of Loki, believed to contain one of the Infinity Gems, was also in Von Strucker's possession.

There is also a rumor that Captain America is poised to take the reins of The Avengers from Tony Stark. Marvel is really establishing the Star-Spangled Hero to be their flagship (pardon the pun) hero, while Iron Man finishes (or is already finished with the third one?) and Thor has his own Asgard agenda.

"Captain America: The Winter Soldier" releases April 4, 2014,  "The Avengers: Age of Ultron" on May 1, 2015.