To the later point first: many of the other choices (Mar-Vell, Quasar, Nova) are human or Kree, and wouldn't necessarily go unrecognized by the Xandarian scans.).
Right, but they could easily explain that away by saying that (whichever's) power source were not known to NOVA and affected Star Lord's pysiology.
Effectively, if it was Mar-Vel, even though he was nothing more than a Kree, the radiation that he was exposed to, that allows him to absorb solar energy; or the Nega-Bands themselves, are unknown to NOVA.
If it was Quasar, even though he is nothing more than a human, the Quantum-Bands that give him his Quantum Energy powers are unknown to NOVA.
If it was Nova, even though he is nothing more than a human, it's the force that gives him his powers that are unknown. Of course, in the comics, it's actually the NOVA Force, so that would likely be known to the NOVA Corps, but they could make it where Nova got his powers from something else, if they wanted. I do agree that Nova is less likely than many of the other options. I actually included Nova only because of his connections to the NOVA Corps, and because I would love to see things develop to the point where we have the New Warriors.
Just imagine this...
In GOTG2, Xandar is attacked and Nova Prime gives a mission to Rhomann Dey to take something and protect it. He takes off in a NOVA Corps ship and commands the crew to go. They become attacked by say... Nebula. Damaged and about to be destroyed, they are able to go into hyper space and end up near Terra.
If this story plays into the full movie, then they follow him down to Earth. If not then it could be an after credit scene. Let's go with the first one as I would love to see Nova involved in the storyline.
With the NOVA ship and crew destroyed, with the exception of Dey (maybe he's protected by the energy source of what he is protecting), he crashes to Earth. He lands in Hempstead, Long Island and is found by a kid named Rich. Rich is given/exposed to the energy of the item that Dey had. Maybe Dey dies (which would be too bad as I liked him in GOTG).
Richard Ryder becomes Nova and dons the suit given to him by Dey (or the energy source gives him the suit). Anyway, we now have Nova. Maybe Nebula comes after the power and/or something calls him to Xandar. He gets caught up in the whole battle against Nebula or whatever bad dude the Guardians will be up against in #2. At the end he wants to go home and be a kid. He has to give up his powers to the NOVA Corps and he's sent home.
Post Credits scene:
Locaton: Earth. Hempstead, Long Island. An alley, outside of the kitchen entrance of a small diner.
Time: Late evening. Closing time. The alley is dark.
Richard Ryder opens the door carrying a bag of trash. He locks the door and turns toward the dumpster.
Suddenly a metal grapnel line comes out of nowhere and wraps around his feet. He is taken from his feet, the trash bag falling to the ground as he finds himself being pulled upward.
At the top of the five floor building Nova comes face to face with his attacker, a main dressed in some sort of black colored battle suit, wearing a black helmet with red accents.
As the man grabs Richard by the neck, hoisting him upright, the grapling line retracts into the assailant's guantlets. The black-armored clad figure continues to hold Ryder over the edge of the building, lifting him so they are face to face.
"So you're the Human Rocket, huh?" states the dark figure, in a deep, vibrato voice.
He suddenly opens his hand and Richard Ryder finds himself falling.
The camera follows him as he falls, focusing on his look of terror. Suddenly Richard Ryder's eyes start to emanate a blue energy and Richard smiles.
Fade to black.
Actually, the New Warriors could make a pretty good tv show, if done right.
To the later point first: many of the other choices (Mar-Vell, Quasar, Nova) are human or Kree, and wouldn't necessarily go unrecognized by the Xandarian scans.
To the first point, I believe Yondu was also seriously different from his comic character, so it's not like they can't change things. For all we know, (WILD SPECULATION ALERT) perhaps Yondu and Starhawk really
are refugees from a dystopian future, and they got sent back to the present using the Time Gem, to prevent the destruction of the universe that occurs when Thanos gets all the gems. But they went back too early, and ended up parting ways. Yondu considers Starhawk a jerk, so he decides to abandon the mission, and falls in with the Ravagers instead. But he's proud at the end of GotG, when he realizes that Starlord didn't hand over an infinity gem to the Ravagers. Yeah, I know, its crazy...

(after all, GotG2 is more likely to be about Warlock...)
But yeah, I still think it's probably Starfox.
Actually, I kind of like this idea of yours. By going with a future storyline like this, where both Starhawk and Yondu originated, you could include other future characters like:
Vance Astro (could lead to Marvelboy as a member of the New Warriors)
Captain Charlie 27
Martinex T'Naga
Niki