Disney fansite Stitch Kingdom has a rundown of the cameos on the movie (including who Stan Lee and Nathan Fillion are playing) and points out that the creators of Howard the Duck are in the film’s credits:
But saving perhaps the best for lastâ€¦ weâ€™re not exactly sure what it means yet â€” if anything â€” but while he couldnâ€™t get himself a 25th anniversary Blu-ray (and not from lack of trying), Marvel legend Howard the Duck is somewhere to be found in the film. This inference comes from a line in the end titles that credits co-creators Steve Gerber and Val Mayerik. Whether itâ€™s just a film/comic Peter Quill was familiar with before he was abducted from Earth or weâ€™re about to have a new entry in the Marvel Cinematic Universe is just about anyoneâ€™s guess.
As they point out, it could just be a comic in the background or something like that, but how awesome would it be if it’s something bigger than that?