» Life Model Decoys
It's already been established that there are synthezoids and life model decoys in the Marvel movie universe. Life model decoys have been mentioned several times now, in both "Iron Man 2" and "The Avengers," while synthezoids were briefly discussed in "Captain America: The First Avenger." Since there are already fully functioning androids that exist, it seems only fair to assume that The Vision is just around the corner.
» Lack of Emotion
If there's one thing people remember about Phil Coulson, it's that he rarely showed any signs of emotion. Always appearing calm and collected, even after being stabbed through the chest by Loki, Coulson is already android-like in that regard. The fact that he had the foresight to advise Nick Fury to use his untimely death as a means of motivating the team only adds to Coulson's cool and calculated nature.
» Stark Tower Security
Towards the beginning of "The Avengers," Agent Coulson somehow manages to bypass all of the high tech security that Stark Tower has to offer. How is it that Coulson is able to waltz right in to the home of one of the richest and smartest men in the world? Well, perhaps he has the ability to manipulate his density. You know, making himself intangible and what not.
» One Good Eye
After the death of Coulson, Nick Fury states that he "lost his one good eye." While this is clearly a play on words, being as Nick Fury wears an eye-patch, it could also be a reference to Coulson's codename: Vision. In the field, Agent Coulson would act as Fury's eyes allowing him to see anything and everything that Coulson sees firsthand. Get it? Vision? Eyes?
» Jim Hammond
The only thing that Agent Coulson ever showed the slightest bit of enthusiasm for was Captain America. Jim Hammond, the original Human Torch android, fought alongside Captain America on several occasions back in the '40s, according to comic book lore. he body of Jim Hammond is also the original synthezoid on which The Vision was based. It's a stretch, but that could serve as yet another clue that Coulson will come back in android form.