This does not sound very exciting, because Venom has always been part of Spider-man's life and universe. So it's not that unusual for them to cross paths. It's just like Batman and Catwoman or Batman and the Joker. They're all bound to meet in Gotham City, right?
So is the same case with Spider-man and Venom... they were together -- they had the same history -- they're from the same place -- they're from the same "Spider" universe.
Avengers is different because we got 4 characters from very diversified universes come together -- Iron Man from the technology world, Thor from the mystical world, Hulk from the genetics world, Captain America from the world ware II era...
Forget the boring Spider-man Venom crossover.
Here’s an even better idea (well actually it’s already happened in the Marvel comic books, so now they can make it into the “Movieverse” if they choose to) -- they could do Avengers vs X-men.
Since the X-men franchise is almost (or is) done at this point, they can use it one last time. Even a trilogy would do.
The plot could be something like this: some right-wing political group passes an anti-mutant bill which prohibits mutant existence. Some angry mutants (maybe headed by Magneto) see the humans as a threat and start an assault on humanity. Humanity’s last hope now lies in the Avengers! Then so it begins!
And oh, Spider-man can even make a guest appearance too (siding with the Avengers of course)! Explosive!