I'm not even going to be elaborate, cite sources, or further discuss on this subject. This is going to be very quick, and straight to the point. This has been a hot topic on forums since both the info leak and release of Guardians of the Galaxy. People are thinking that because Lucasfilms made the first Howard the Duck, and now that Disney own's both Marvel and Lucasfilms, that the cameo is hinting at a reboot of the character.
Howard the Duck at the end was strictly for the fans of Marvel. An average Joe is not going to recognize him (unless he/she grew up in the 80s), just like they wont recognize the Guardians themselves. Howard is in the after credits scene to pay respects to his creator, Steve Gerber. Steve Gerber was the writer of some the Guardians of the Galaxy stories that were featured in "Marvel Presents" back in the mid 1970s. Consider him the guy who brought the GOTG to regular publication after a few guest appearances in the early 70s. Many fans of the original [Guardians of the Galaxy](movie:424073) consider him, along with Jim Valentino, to be the most important contributors to developing the Guardians.
This is literally the answer. Even in the comics, Howard the Duck was never a Marvel character given weight and character development. Howard came from Adventures into Fear #19, in Man-Things short story, he has been in various comics - Spiderman team-ups, the Fearsome Four- with She-Hulk, Dr.Strange, Ghost Rider, Savage Dragon, had his own comic strip in late 70's in a few newspapers, Marvel Zombies (2010) , the list goes on but you get the point. The only reason why the 1980's film was even made was as a comedy movie and nothing else.
Howard the Duck is not going to get his own film. Just like the comics, his role in the after credit scene was simply: A cameo, a character to pay tribute to both the creators and nostalgic comic fans, and nothing else.