Soars into battle,
When I first saw [Guardians of the Galaxy](movie:424073), I was blown away, just like many of you. James Gunn took the classic Marvel formula and made it work in the cosmos. One of the things that impressed me the most was the number of ways in which the film's loose ends could be tied up. Probably the most talked about loose end in the film is the issue of Starlord's father. Nova Prime refers to Starlord's father as "something ancient," which could refer to countless characters within the Marvel Universe. It is the context of the Guardians' introduction to the Marvel Cinematic Universe that makes me suspect Starfox, the brother of Thanos, is the baby daddy.
Intergalactic Don Juan
It's well known that in the comics, Thanos' brother Starfox (aka Eros) is a big-time philanderer. He eventually faces trial before the living tribunal for seducing She Hulk. His charming, womanizing ways would fit well with Mary Quill's story about a glowing figure who seduced her and got her pregnant. They would also fit with Yondu's description of Quill's father as a jerk.
Nova Prime describes Starlord's father as "something ancient." Anyone familiar with Thanos' background knows that he and his brother Starfox belong to a race known as the Eternals, an evolutionary offshoot of humanity engineered by the Celestials to be capable of manipulating cosmic energy. If Starlord's father were an Eternal, it would explain why was able to use an Infinity Stone to defeat Ronan the Accuser.
A final reason I think Starfox is Starlord's father is that cinema and theater are full of uncle-nephew rivalries, and Thanos being Starlord's uncle would make for a perfect rivalry. While some have suggested that Adam Warlock could be Starlord's father, I think the idea of Starfox being his father would he more dramatic. So, who do you think is Starlord's father?