The post-credits sequence of the last two Marvel films have teased not the immediate next film in that Cinematic Universe, but rather the one after that. "Thor: The Dark World" teased "Guardians of the Galaxy," while "Captain America: The Winter Soldier" teased "Avengers: Age of Ultron".
Therefore, the upcoming "Guardians of the Galaxy" should logically tease "Ant-Man". However, with that film delayed and probably not shooting before 'Guardians' is completed and printed, there's no real time.
So what might they do instead? Latino Review reports that they think another character will be teased during a post-credits sequence, one that could potentially be played by a familiar face.
POTENTIAL SPOILERS AHEAD
The site reports that the Human Rocket himself, Nova (aka. Richard Rider), will make an appearance. They can't confirm, but they guess he may be played by "Firefly" and "Castle" star Nathan Fillion.
Fillion has appeared in both of director James Gunn's previous features, so the inclusion isn't a big shock - especially as the actor already teased he makes an appearance in the film two months ago.
The report says the character will only be seen during the post-credits scene, and could be wearing a helmet so, like Thanos, he could be recast at some point down the road. The site hasn't gotten confirmation at this time as to the report's veracity.
Essentially Marvel's version of DC's Green Lantern (though there are differences), Richard Rider is an unsuspecting human who is recruited to become member of the intergalactic police force known as the Nova Corps. As a member of said force, he gains superhuman abilities.