Well, as a massive comic book fan, the two big ones missing from the movie are Spiderman and Wolverine. They're both big fan favorites, two of the biggest, but the problem is that they're both owned by other production companies. Now, there have been cross-promotional deals in the past, so it's certainly not impossible, but I would say it's unlikely. It would be pretty neat if people went to see the new Spiderman movie next month and in an after-the-credits scene Nick Fury shows up. People would go freaking crazy for that.
I've heard that there have been preliminary for a Heroes for Hire movie (Luke Cage and Iron Fist), which would eventually branch out into them joining The Avengers for another movie. In the comics they're two of the featured members of the second Avengers team, the New Avengers (there are a few Avengers squads in the comics), which is more street level focused, and not so much built in the stop-the-alien-invasion mold.
I'd love to see a high school aged Spiderman joining The Avengers if they can negotiate the rights to make it happen (does anyone write better teenage heroes than Joss?).
Well, the thing with Spider-Man and Wolverine being big fan favourites...The way I understand it, they aren't big fan favourites in regards to being with the Avengers team. I don't see it working out well on screen, just as I wasn't a fan of those two being in the team in the comics. I would say it's more than unlikely that they won't be added to the current Avengers line-up; I'd say it's nigh impossible. Marvel would want to be able to do their own versions of both respective superhero flicks, with their own tie ins, before bringing them into the fold. Marvel will not own those rights again, at least not without tossing a substantial amount of money at both Sony and Fox. Reboots of several FAILED franchises are being planned simply so those companies DON'T lose the rights (Fantastic Four, Daredevil, etc.,) so I highly doubt they are even going to consider giving them up at any point.
Spider-Man, IMO, works better as a standalone character. Matter of fact, I'm still not sure if they can successfully bring him to the screen. I hated the first Spider-Man trilogy, and while I'm going to go see TAS in theaters when it comes out in July, I'm not convinced it will be amazing.
And Wolverine...Well, as far as the movie versions are concerned...I'm sorry, I don't want "WOLVERINE!!!...'n friends". Or, actually, sorry, more accurately...I don't want "WOLVERINE!!!...'n friends" number four, as we got the first three with the X-Men movies. Another thing to consider is that, even if they were somehow able to get back the rights, and do their own movie....Hugh Jackman *IS* Wolverine to general public. If Marvel were to do a reboot, they would undoubtedly recast, and who knows if that would be received well.
I would not be appeased, as a fan of movies, OR as a comic fan, if these characters were brought to the Avengers on film, just as I wasn't pleased when they were brought to them in the comics.
Another thing to consider when you mention 'fan favourite,' what type of fan favourite are you referring to? Keep in mind that none of the characters currently in the Avengers movies were 'fan favourites' for the main populace until their respective movies were released. Pre-movies, if you had asked the average person on the street who Iron Man was underneath the suit, most of them would not have been able to give you an answer. Out of the characters currently in the movies, the only one that I would say the average person knew about was Captain America, and even that is stretching it a bit.
I'd personally like to go with people that are unknown to the general public. I also think we need more female superheros in the team as it stands. Wasp is a popular choice, and I wouldn't be against her, as I like her character. The only issue with bringing her on board is that she likely comes hand and hand with Ant-Man, and I do NOT see that transferring to screen well at all.
Spider-Woman (Jessica Drew) could bring an interesting dynamic to the group, I think.
I also wouldn't mind Ms. Marvel or Scarlet Witch. Bring on the female superheros, I say.