Re: Spiderman in the Avengers "world"?
I want Spidey to exist in the MCU, but he doesn't HAVE to be in the Avengers, or rather, a full fledged member.
Treat it like the comics do it and have him be standalone, only joining the Avengers when the threat really calls for it or his participation in the story in necessary.
And as far as the likelihood of it, I'd say there are pretty good odds going for it right now.
Marvel and Sony are on VERY good terms with each other right now, and both sides have shown willingness to not only to tie the character to the MCU (Whedon and Co. wanting to use Oscorp and Oscorp tower, with Stark/Avengers tower likely appearing in ASM; Avi Arad and Matt Tolmach stating openly their willingness in such a venture), but do it in a way where Spidey can stand on his own, separate from the largely interweaving Avengers cast of characters.