On top of the aforementioned customization options, we do know a few details about how gameplay will work in Marvel's Avengers. From the description that popped up on the E3 website (before being promptly deleted), we know that the game is envisioned as "an epic action-adventure that combines cinematic storytelling with continuous single-player and co-operative gameplay."
This mesh of single-player and multiplayer options has certainly caused a bit of intrigue. Could Square Enix be planning Marvel's Avengers to be a big online, live service game like Destiny and The Division, where you can either tackle missions solo or team up with mates to complete tasks together? Certainly, the concept of "continuous" gameplay points to an online game world.
This definitely sounds like a cool strategy, allowing the game's marketing team to target both superhero fans and regular visitors to hugely-populated online gaming worlds. Those fandoms combined would make a very broad church, and the marketing hashtags would write themselves. #AssembleYourTeam and so forth.