(Be warned, this could be a potential spoiler, so please, if you don't want to take that risk, don't read on. I'd hate to spoil it for anyone).
That question is something all Marvel Movie fans want answers too. The top contenders at the moment include Starhawk, Adam Warlock, Starfox, Black Bolt and even Thanos! But I have a theory I think makes a lot of sense:
Uatu the Watcher.
Now you might be thinking that doesn't make sense because Uatu seems to be owned by Fox, but it was never confirmed. So here are the clues:
Uatu is apart of an extremely ancient species.
He is extremely powerful and has the power to project "light".
His DNA is strong enough to contain the power of an Infinity Stone.
It also makes sense why he had to hire Yondu; because he was stripped of all his power for breaking the rule not to interfere with Earth and was banished, so he was unable to return to Earth.
He cares for Earth and it makes sense that he would conceive a child.
He has an angelic appearance.
He can also become human size.
Plus, Uatu has had many encounters with lots of Marvel characters, so he would tie into the universe perfectly.
Makes sense, right? Of course, it may be still likely that Fox has the rights to him, but they really don't need him, because they've got Kang (who they really don't deserve) and numerous others.
Enjoy the rest of your day (you're welcome).