They'd have to rewrite the history of Vision seeing as how Hank Pym aka Ant-Man created Ultron, the ultimate supervillain android, who in turn created Vision to kill The Avengers and ended up changing his mind.
If they're going to turn Coulson into Vision, that'll definitely be a three or four movie build up. They'll need a cosmic character with that much power to defeat ______, anyway. They could, however, just skip Ultron and have Pym create Vision from the original Human Torch design(red suited guy in cylinder display at Stark Expo in Captain America).
My guess is they could keep Coulson around by making a "Life Model Decoy" of him. Stark did that fake voice message for Coulson in The Avengers and mentioned himself to be an LMD, so it's possible. It's also not a cop-out by bringing in an android to replace a once human character, as opposed to just saying Coulson didn't die. It's like having him, but not having him.
Also, here's a couple things bugging me about Avengers and the 5 movie arc it took to get there:
1. Why no War Machine? He was better prepared for a battle like that than either Hawkeye or Black Widow.
2. What the hell happened to Tim Blake Nelson's "Samuel Sterns" character and his subsequent turning into The Leader in The Incredible Hulk?
3. Also, what happened to Stark talking to Ross about the Initiative at the end of The Incredible Hulk? It's clear that storyline was used, yet two big parts of it have been noticeably absent.
It seems to me, after learning that Whedon picked the ______ cameo at the end of Avengers and basically created the arc for the next set of films, that Feige doesn't have a "clear picture" of what's going on. He says he's mapped out this storyline in his head for years, yet there's quite a few things being left out or decided on the fly.