This is what I've been dreaming about lately.
The gimmick here, game-wise, is that the Alliance is a pool of characters that all the players can pull from on a session-by-session basis. No character belongs to any one player; they're all shared equally. So, of course, the characters would all have to be created cooperatively by the players. And that's where the character creation sheet at the right comes in.
Actually, though, I'd start by discussing with my players what kind of tone and tropes they're interested in for the game. We'd try to establish a target seriousness level, and come up with some example media. Then we'd talk over the setting a bit, see what kinds of components people are cool with (magic? aliens? time travel?) and just how "superhero-ish" characters should get (costumes? spandex? masks? code names? secret identities?).
...And then I'd hand out the character creation sheets, two to a player with a pile of extras. I'd explain all the different bits: "Purpose" could be something along the lines of "glory hound" or "With great power comes great responsibility." "Relationship" could mean a romance or rivalry with another Alliance member, or it could be a civilian NPC friend or family member, or even a recurring arch enemy. "Weakness" might be something like "poisoned by kryptonite," but it could just as easily be "hardened alcoholic" or "dangerously short temper".
Anyway, the deal with the character creation sheets is that each player would write something on their sheets, and then pass them off to other players to write some more stuff on, with everybody freely sharing ideas and talking openly about what they're doing. Hopefully, at the end of the session, we'd have at least two filled sheets for each player, and everybody should have some idea about the whole array of characters they'd outlined.
Then, I'd before the next session, I'd do the actual game mechanics side of building all the characters. I'd almost certainly use Mutants & Masterminds, in spite of the mention of "aspects" on the character creation sheets. I just really dig aspects, and think they can be imported from Fate into almost any RPG system. Also, I think they could be employed here to encourage the kind of ensemble-cast soap opera effect that tends to arise in long-running superhero team comics.
Part of what I'd be hoping to achieve with this kind of character generation is a player mindset that doesn't regard the PCs as representations of the players so much as shared game pieces. As such, they should be easier to sacrifice. Dramatic PC deaths would be a really cool and genre-appropriate thing to work into the game. I'd even like to incorporate some variant of Ryan Stoughton's death flag mechanic to encourage it. (CliffsNotes version: Normally, no PC dies. But you can trade this plot immunity for a temporary powerup, thus very possibly going out in a blaze of glory.)
Will I ever actually get to run this game? Dunno. Might not work too well with my local crew, but I'm edging closer and closer to running a Skype game of some kind, and I know I could find the perfect crew for this game once geography isn't a factor. So maybe. Some day.