^ Even if they change the who from J'Sonn to someone else, it doesn't necessarily mean that that person couldn't be "royalty" or the leader of some world or other people.
Adam Warlock doesn't seem to fit, IMO, though he is on the short list of possibilities. The problem with him is that he is going to need to have his own story. That story could be introduced in GotG2, or it could be in Avengers, or even in Doctor Strange. But making Adam Warlock Star Lord's dad would come off a bit boring, unless they totally change the character of Adam Warlock, which I truly hope they don't...
After the first film, I think one thing we should rely on is that whoever ends up being Quill's daddy, it'll be fun. Sure, I'm sure whoever it is truly loved Quill's mom, but it will still need to be fun in order to match all of the things we know, including Yondu's comments.
Could it be Yondu? Sure, but I really hope not. Although that would be an ok story arc, there are so many better ways to go.
Star Hawk is another possibility, but he's such an unknown and the only real interesting things about him would not really be possible to use in this movie-verse (male/female personas/Hawk God powers/being the son of Quasar and Kismit). One reason I could see them using him is the storylines that had him teaming with the Guardians against Korvac, who would make for a really good villain, and possibly the Badoon. Truth is though that they don't need Starhawk in order to use Korvac or the Badoon. Actually, the Korvac background that they used in the animated The Avenegrs Earth's Mightiest Heroes would work very well...
As far as the Terrigen Mist recommendation, now that we've seen that in Agents of Shield I'd have to say that that would not work, so I'm thinking it won't be an Inhuman (that story will likely come from what we see in future episodes of Agents).
Some of the other choices, like a Cellestial or such, is so far out there it's hard to give it credit as a possibility.
And that leaves someone like Star Fox, who is related to Thanos, which would make Quill related to Thanos, which would fit the storyline very well. Also, his background and powers match just about every hint we were given regarding who Quill's dad is:
Also, wouldn't a guy who has the ability to make all of the hot chicks desire him (one of the many powers of Starfox, aka Eros) be thought of as a "jackass" by Yondu?
'Quill, I am your... Uncle"!!!