Going back to my secret wish of Coulson being Mar-Vell, I would love it if it were the Kree.
Imagine this, a specific unit of the Kree sent in to scout out earth after being alerted to the presence of the cosmic cube, a source of power and energy that could be used in their war with the Skrulls. These Kree have infiltrated as sleeper agents and have bided their time until the cube is within reach.
The end conflict could be their escape plan, some sort of devestating doomsday weapon to distract as they escape with the cube. Anyway, movie plays out, they get thwarted but warn the coming of a great war and they shall be destroyed.
Then, end credits scene, Fury talking to Coulson, Fury leaves and Coulson powers on Mar-Vell style :P
I just think it'd be totally cool if, after SHIELD and earth think the Kree are a serious threat, one of their allies turns out to be a Kree warrior. In Avengers 2 he could get incarcerated by Fury and interrogated. Then, the Skrull or Thanos can come to earth and Mar-vell becomes an ally in the end. I could definitely see something like that working really well and blowing the whole Marvel cosmic universe wide open.
Pure fantasy though, wishful thinking :P