Now although this is speculation and not confirmed by James himself or any of the cast members, I think this is a huge Easter Egg towards all 6 Infinity Gems.
I think all these people are fighting for the Infinity Gems and I'll explain why I think so.
Now for those who haven't made the link yet, here is Loki's Scepter heavily hinted and I think confirmed to be one of the 6 Infinity Gems. As seen in the Avengers comes for the Chitauri.
Last seen: Baron Von Strucker's research facility with [Quicksilver and Scarlet Witch](movie:1081109).
Next up is the Aether.
This was the powerful weapon wanted by Malekith the Accursed to continue with the Convergence of the 7 realms and wanted to plunge the Universe into Darkness.
Last seen: In the Collector's collection, however we do not know what has happened after the explosion in Guardians of The Galaxy.
Now I know what you are thinking, "what about the Tesseract? that's blue too!" Suspend your pre-conceived thoughts for a moment and take a look at the picture. There is a greenish colour there (lighter than the other green) and I think this is our best bet.
Last seen: In the crypt in Asgard. As Asgard had one gem already, they did not want to risk having two in the one place.
Also in the portrait on Morag we have this indistinct colour inbetween the blue and the green and I think our best bet is The Orb seen in my personal favourite [Guardians of the Galaxy](movie:424073).
Last seen: On Xandar, held by the Nova Corps.
Now as we all know we still have two more infinity gems to go before the Mad Titan himself, Thanos, gets his grubby paws on them!
So is this telling us the next 2 Infinity Gems are going to be Yellow and Green!
I believe so, but let me know what you think below!