[@Su Yin]:

[b]Guardians of the Galaxy[/b]: While the Guardians are not off the table, the risk with them is that the majority, if not all of the characters currently in the game, are based on Earth.  Therefore, it could become lonely out in space without people to interact with.

[b]Spider-Man[/b]: He's already taken, so he would be off the table.

Between Maria Hill and Agent 13, I would say go for Agent 13.  Captain America (Steve Rodgers) will be becoming important in season 2, along with some other SHIELD characters (like Nick Fury and Hawkeye).