Well, they could go the alternate universe route and somehow have Peter Parker transported to the MCU.
They could even make it the same universe, and explain away all those construction rigs that Spider-Man used to swing on to get to Oscorp tower as being there to rebuild a certain part of the city after the Chitauri battle in Avengers. It would feel a bit forced, since you'd think a massive alien invasion would be mentioned at some point, but it could work, I guess.
And it would be stunningly cool to see Spider-Man interacting with and helping the Avengers. The banter from him aimed at the other Avengers characters in a first meeting would be hilarious.
From a business standpoint, after ASM2's box office falling short of projection, it would make perfect sense for both studios. Marvel obviously would love to have Spidey's rights back, but I do think they'd be willing to settle for working with Sony to combine the franchises. And Sony most definitely would be onboard, since Spidey crossing over into the MCU would most likely give their ailing Spider-Man franchise a huge boost in the arm in terms of fan interest and box office returns for future Amazing Spider-Man sequels and the various spinoffs they have planned (Sinister Six, Venom).
I've read that Sony has been having financial trouble and may look into selling off properties they have the film rights to, so they may be in a financial position where such a crossover would help them greatly (since I doubt they'll sell off the Spidy franchise to Marvel due to it being one of the franchises they own with the most promise in terms of economic gain).
