The following is a ramble in which I sort of "walk-and-talk thru" the Starfox option. Maybe some detail-spoilers from the movie, but mostly just nerdy Marvel Universe (comics) talk. Not quite final or buttoned up, but hopefully I'll revisit before the second movie hits theaters. =)
Still, let me play Starfox out a bit more… If Eros is the brother of Thanos in the MCU I think his power set and levels would have to be pumped up. I always thought he was shortchanged in the MU as Thanos's brother. Sure, I get the Thanos/Eros dichotomy, but man, one becomes the champion and consort of Death, the other plays space-adventuring Casanova?
Eros should be pumped up as a champion of Life, someone/thing akin to Mar-Vell or Adam Warlock, y'know? Of course, Adam's cocoon in the Collector's Collection would indicate that the MCU doesn't need another Adam Warlock-type. but let's assume that cocoon doesn't hatch anytime soon. Or maybe that it houses The Magus, who is actually a non-Thanos opposite to Eros, not Adam.
Okay, so Eros's space philandering leads him to spend time on Earth in the 70s. He falls hard for Mama Quill and they make a home together until he learns that his brother has destroyed his homeworld (called Titan, but not Saturn's moon, somewhere farther aspace) and killed most if not all of their family. He knows Mama Quill is pregnant, but to protect her, their unborn child, and the Earth he has to leave and go into hiding. He promises to return when it's safe and do whatever he can to care for their child when he can.
So, thanks to his cosmic senses, champion of life Eros would know when Mama Quill is ill and passes on, and so contracts Yondu to pick the boy up.
I can imagine Captain Marvel or Adam Warlock type working with Yondu in many situations while keeping a low astro-profile, and could easily see space biker gang leader Yondu thinking of a stick-up-his-bum champion of life like Peter's dad as a "jackass." Yondu may have legitimately believed that he was saving young Peter from a boring life as a straight-laced do-gooder when he decided to keep him for the Ravagers instead of delivering him as contracted.
I wonder what Yondu's story was to Peter's dad about that. Maybe he just never showed up for a scheduled rendezvous/hand-off, leaving his client to believe that he'd failed to find Peter or just wasn't interested in the job. Of course, Eros, champion of life, knows that Yondu kept him, just as he'd planned. With Thanos still at large, Eros is still on the run or in hiding, so Peter would be safer with the Ravagers than by his side and with Yondu as his mentor and guardian would learn to become a capable cosmic scoundrel and survivor.
If Eros is Peter's father, does that explain the scan that describes his heritage as "something ancient, that we've never seen before?" Perhaps. If we place Titan in a remote part of MCU space, and their population as small, or intentionally hidden/isolationist. Sure, why not?
Or… We could place Thanos's Titanic genocide much deeper in the past, with Eros as an only or one of the few survivors, and already in hiding when he arrives on Earth and falls for Mama Quill.
In any case, Eros's bloodline could read as "something ancient" and unknown to Xandar.
Eros could certainly appear as an angel, composed of pure light. That could easily be literal for an extraterrestrial champion of life (imagine Mar-Vell or Adam as Eros), and metaphorical, for an Eternal of love, right?
So, sure, an adapted and upgraded Starfox/Eros in the MCU could definitely be Peter's dad. Does it mean anything significant to future plotlines and conflict? Well, they could try to play up the relationship to Thanos, but why bother? I don't see a lot of fun coming out of snippy dialogue between the two around family. They don't have enough common experience. The one strategic play I do see is that Peter's rare Eternal blood (and blood-relation to Thanos) could allow him to endure or use a device or plot concocted by Thanos to destroy his enemies because it would be keyed to his physiology. Peter's a demi-Titan, so he'd probably die, but hey, a hero is up for that, right?
They really can't call him "Starfox" tho, can they? "Eros" sort of works next to Thanos, but even that is kind of a tough sell. Maybe I should find/replace "Starfox" and "Eros" with "Captain Marvel" or "Starhawk" everywhere in this write-up instead?
Keep on keepin on~