Agent Phil Coulson, with his amazing power to bypass any and all emotion in the midst of the greatest danger, was a character Marvel created from nothing for their cinematic universe. For the three of you that have yet to see The Avengers, he also is the member of the SHIELD team that Whedon decides to impale.
However, there's been lots of speculation that because he is contracted to do more films (I believe a total of six, but that may not be correct), many believe that he will return as the legendary hero "Vision," as created by Dr. Pym in the comic mythology.
And might I say, "Holy #*$& that's awesome."
But I don't believe it true for the following reasons:
Agent Coulson dies in front of Director Fury while they're, as far as we know, the only ones in the room (it can be argued that the medical team is already there, but they seem to only do their job once Fury and Coulson complete their conversation). And Fury is torn up about it. His expression, even after Coulson's death, is one of genuine hurt and anger. If Coulson is a Life Model Decoy, why the sob scene? I understand that Fury is manipulative, but he also does sniff at what's easily fixable. If Coulson was a Life Model Decoy or even an Android, that's an easily fixable situation. He still could have used that death to bring the team together, but I don't think it's out of the realm of possibility to imagine that he still could have done that nonetheless. Besides, Coulson was dating a cellist. Not necessarily an Android thing to do (unless of course, that's why she moved back to Portland).
Is Phil really the top choice to turn into Vision? Obviously, Coulson is a major player in the universe, but he's primarily Fury's scout. His only other bit of action from Avengers is his one-shot video "A Funny Thing Happened..." Remember all the agents that were killed because Loki was adopted? It can be assumed a plethora of those men and women were also field agents suitable for this "Vision" transplant. Yes, Coulson has a higher ranking, but in a way, it would be like Captain America all over again. And after the Hulk fiasco of attempting to recreate super soldier serum, is SHIELD really willing to risk it?
...I take that back. SHIELD probably would.
Joss Whedon isn't known for bringing characters back unless everything is in already in...excuse the pun...his vision. He brings Cyclops back from the dead in his Astonishing X-Men run for a quick and minute plot development, but he sacrifices Kitty Pride for the greater good. Wash from Firefly wasn't able to catch and enormous wooden stake. The only thing I can think of that's somewhat similar to this work is that Whedon has stated he wants to bring Penny back in DHSAB as a robot. But also remember, Whedon doesn't know if he'll direct an Avengers trilogy. He's currently torn. Setting up a character like this for his vision means he's already committed; which currently he has not. I doubt he would be able to convince Marvel to add Coulson on and add another team member to the roster that's going to take up a large chunk of time and back story. He was already given a hell of a lot of creative freedom, but setting up that character for that and then abandoning the project is careless. Whedon's a man who knows character development after working in TVLand for so long. He doesn't play that way.
And then, there are the few obvious ones:
Coulson now exists in the comics world, and is not Vision.
Pym has yet to be introduced in the MCU.
Thanos, not Ultron, seems to be the villain of the trilogy, pulling the strings.