My impression was they were going to do origin stories for the other key Avengers (Nic Fury, Cap. America, Ant Man (I'm assuming a straight up comedy, how else can you pull that off?)) maybe even a villan or two, then one big huge Avengers movie. Meanwhile, the Wolverine and Magneto movies, the Silver Surfer movie, etc...
But their Avengers plan makes me think they inked the headline stars of the origin movies to be in the main one (and continue to make cameos in others). They are making a very low-risk gamble putting out all these origin movies that bank huge: even if the Avengers movie/(series?) doesnt come off, they'll have a back log of big money films that promise the worlds greatest never-made sequel.

Meanwhile, Sony paid Marvel for the rights to Spiderman and there is a script being kicked around, god knows why or who will want to go through the motions with it.