I don't think you're reading too much into it. At all. I think the "inhuman" comment, taken with the "III" planet (Earth) iconography, and the starburst in space, all point towards the Kree and Inhumans, as jaqua and others surmise. I'm not entirely sure the implication is actually that Coulson is Mar-Vell, but I'd be willing to bet anything that he's Kree and/or Inhuman, and has no knowledge of his alien past. It would explain why Fury would be so desperate to resurrect him, and could probably explain some other Coulson mysteries. Remember, SHIELD is still rumored to be using Rhomann Dey as a liaison with Nova Corps in GOTG (a pretty wild rumor; but it came from the same source that accurately leaked that John C. Reilley would be playing that role), so odds are that SHIELD and Fury have a fairly long-running association with aliens (I believe SWORD was even referenced in a recent AOS episode). In other words, I think Coulson is Kree or Inhuman, but he might not be a canonical comic-book character; rather an original one.