The Avengers is undoubtedly one of, if not THE blockbuster of the year. It has a few contenders with the Dark Knight and Spiderman (See a pattern here?), but honestly, this is a movie with so many movies leading up to it that I can't imagine it being defeated at the box office this year. That being said, it almost guarantees a video game to come.
Chances are, somewhere, someone is saying it needs to be based on the movie. This would be the first mistake. The problem with movie based games is that it never really is the movie game. Spiderman movie games always add new villains and situations that have nothing to do with the movie and it feels a bit less movie-related. And you can't very well put only what was included in the movie or you've got a short game and it's a failure by most gamers standards right there.
The group of us that have always known about/read the Avengers is small in the grand scheme of things. The world has been introduced to the Avengers now and they're going to want more. So I propose a game that doesn't hold the restraints of a movie game. I propose an Avengers game voiced by the actors that played each role but in a sort of a sequel, separate from the movie but within the world the movie created.
What kind of features should it contain? Co-op is a no-brainer. Competitive Multiplayer would provide continued playtime past the single player. Should it go open world? Linear? Open world could certainly work, having a floating heli-carrier and all. I'd hope not to see a 'Collect 50 S.H.I.E.L.D. icons around the city" thing. Personally, I hate that. I'd rather have more side missions. Linear works for some players, having a clear and defined destination with bonus levels unlocked, maybe. But I am going to focus more on a few different features.
Characters. Everyone has their favorite, but I have spoken to friends who are fans and have felt a bit more love for the other characters after all the moves leading up to the Avengers and the movie itself. Each character is made for video games and I'll present my case here.
Captain America. He's a fan favorite by many, but how does he work as a playable character? Well, he's got both melee and ranged covered with his shield. I'd give Cap a combo meter. The more hits he strings in a row, the more he continues to improve in combat, maybe speeding up a bit or doing a bit more damage as he goes. He would have a regenerating health meter, of course. After a K.O., he'd have downtime and then pop up with some health and ready to jump back in.
For alternate costumes you've got plenty of options from his WW2 outfit to modern and variants like U.S. Agent and Punisher's variation of the costume. The outfits could change the appearance of the shield, such as the WW2 cap's shield being in the shape of a crest.
Hawkeye. Not universerally known until Thor and the Avengers, he's actually a very nice video game character. Pre-mission you can select a certain load out for his arrows, calling in for ammo drops during the mission when he runs out. There could be a lock-on system for arrows that have multiple projectiles or homing arrows. There are many ways to make Hawkeye an easy favorite in the game.
He wouldn't have a meter other than health, just an ammo count and maybe a timer on calling in ammo drops. His melee should be below par compared to the others, but stabbing a few enemies in the face with an arrow would be a nice little addition. Hawkeye has had a few iterations in costume design and a wide array of arrow types to fire off. He would have less health than most, but would hang back in combat so it wouldn't present too much of a problem, adding in excellent agility (Dodge, etc.).
The Incredible Hulk. How to balance the Hulk? He can't really have a life meter, as he isn't really going to be killed. I suggest a rage meter that fills as he is hit and as he does damage. The lower the meter, the easier it is for him to be knocked back and the longer he stays down before getting back up. When the meter is full, an overdrive-like mode comes into play and it slowly drains. During the draining period, he cannot be knocked down and does double (triple?) damage. Perhaps if he is knocked down too much or remains low on rage too much be shifts to Banner for a time where he is essentially useless and is knocked down at the slightest attack.
There are plenty of costume variants for the Hulk such as Mr. Fixit, Ultimate Hulk and hell, give him a purple pants variant. The Hulk is easily the simplest character to play with the highest potential damage output, making him a character you're not as likely to be upset to play in exchange for your favorite character someone else might have chosen.
Iron Man. Cue the AC/DC. Iron Man is probably the most customizable character of the bunch, with all his variant suits with various strengths. His alternate outfits would behis alternate abilities. There could also be specific customizations per variant suit he is using, but overall, you'd pick the suit for the job. Alright, you got me. I just want to play in the Hulkbuster suit.
Iron Man would have flight, ranged and melee. Iron Man would have an energy meter. The more he fires off the big guns the more energy he uses up. When it's low or used up, you'd have a command to return to charge back up or replace his core. There would be a timer on it where you'd be out of play, so maintaining your use of power would be essential to being part of the fight.
The Almighty Thor. You've got the flight and the hammer, covering both melee and ranged. After filling a meter of damage he's done, Thor could charge his hammer with lightning to either shoot lightning all over the place like a madman or charge his hammer with extra damage for a limited time. Thor isn't immortal so I'd imagine him getting a health meter, but one that would regenerate and would be fairly extensive. As for varying outfits, he's got a few, including Ultimate and his original look.
So, as I see it, each character would have unique aspect and manners of customization. Progression would allow further upgrades, like damage, speed, durability, etc. The character you choose would determine what aspects are available for for you to upgrade.
The final thing I'd like to touch on is support characters. There is a very long list of potential support characters, or if Marvel would be willing, unlockable playable characters that have been Avengers at some point. Spiderman could be an easy choice, tying into the upcoming movie, using the same guy acting in the movie providing the voice. He has been an Avenger, after all. Other choices could include Ant Man (Hank Pym), Black Panther, the Wasp, Scarlet Witch, Quicksilver, Vision, Black Knight, Beast, Wonder Man, Ms. Marvel, Falcon, Tigra, She-Hulk, Mockingbird, Namor, Moon Knight, U.S. Agent, Any of the Fantastic Four, Spider-Woman, Luke Cage, Wolverine, Ronin, Ares, Dr. Strange, Daredevil, Storm... The list goes on.