marvel cinematic universe - What would have to happen to merge the Spider-man franchise and the Avengers franchise?

The two franchises are held by two separate companies. One, Sony, holds the rights to Spider-man 4-7, and are obliged to continue this line or give up the rights to the movies. The other, Marvel Studios, owns the rights to the Avengers and the subsidiary characters.

Under what circumstances might Spider-man join the Avengers? Are movie rights buys outs uncommon? Could a serenity/firefly-esque loophole be pulled off? Could the two studios 'team up'?

The original reason for me asking this is that Spider-man plays a large part in the civil war comic. With The Civil War movie in the horizon, and the shuffling of executives in Sony after the hack debacle, I thought there might be some news on this.

Answer

Film corporation collaborations are not as uncommon as one might believe. This is especially true when dealing with music or music artists signed to one label that is under the umbrella of another corporation. Granted in those cases the film corporation really tries to pull music and artists from under it's own parent company labels but such is not ALWAYS possible.

Likewise when taking characters it always depends on what the film companies are willing to do. In that case it's always going to come down to money. Who gets what portion. This involves actors, screenwriters, and of course franchise owners.

The best example of this is "Who Framed Roger Rabbit" which gave us the first appearance of "Bugs Bunny" and "Mickey Mouse" on the screen at the same time. And the blood between Warner and Disney over animation was always HIGHLY hostile. (More talent theft than you see between Google, Apple, and Microsoft)

So, while collaborations aren't rare; what they collaborate over can be far more rare.

So, to answer the question, sometimes it's a contractual buyout of the franchise/character. Sometimes it's a character loan.

As mentioned in other responses, after months of deliberation, in February of 2015, Sony and the MCU announced an agreement to allow Spiderman to appear in upcoming MCU films. Details can be found at the Marvel.com website.

As per the original answer; it comes down to two corporations with rights to come to a contractual agreement over those rights. The same situation would be the case with the X-Men, or a Marvel/DC cross-over. The latter, far less likely.