First off sorry if this article isn't as intricate as most others, but it is my first one. Anyways this is my take on how introducing Spidey in The Avengers could make sense for the continuity for Marvel's phase 2 and Ironman 3. Here goes...
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So when Ironman is falling back to earth after the portal to space closes, instead of Hulk jumping heroically and catching him, you could have a quick shot of a red and blue blur catching Ironman AS Hulk TRIES to catch him. For example you could have a slow motion shot of Hulk jumping to attempt to catch him, then do a high speed transition of Spidey swooping him out of the air (hence the speed blur) and hear a few web shooting sounds before you cut to the next scene. Upon the next scene, you could have Ironman hanging inches from the ground suspended by a few webs linked to nearby buildings.
Cut to the rest of the Avengers showing up by Ironman's side and Thor ripping his face mask / helmet front off, followed by Captain America making a quick comment about the webbing and the "mystery man" before being cutoff by Hulk roaring, and then Tony Stark waking up as it happened in the movie. Then Hulk sees the "mystery man" swinging multiple blocks down range and he spontaniously decides to chase him, for whatever reason we do not know. Captain America and Thor then converse something to the effect of " Help me get Tony up./ What about Banner?/ Let him go for now. He'll find his way back, he always does".
Cut to Ironman, Cap, Thor, Widow and Hawkeye eating their schwarma and Tony asks Widow "So where did Dr. Banner go?" and She simply replies "S.H.I.E.L.D. is working on tracking him". Then Tony asks "So who the hell caught me?" and to that she replies "I will debrief you as soon as possible. But can we please finish eating first?"
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The spidey thing could be explained if Spidey was fighting Lizard in another city at the time the alien invasion began, and finished off his out of town business just in time to swoop in and catch a fellow hero. You would'nt ever have to show Spiderman in full, just a blink and you miss it cut of him rounding a corner or even a quick long distance shot to establish the color of his suit.
Anyways sorry for the lenghty post, but I've been wondering if that could make sense or seem like a cool idea. Two thumbs up to the billion dollar beast we call The Avengers. Goodday.