You'd need to have a lore explanation
Comic book fans are strange in some ways. I don't care exactly how Superman's invulnerability works; I can accept it as part of the character ideal. I do care about why there are two or more Supermen in the same place. A full-on MMORPG needs to address this in some fashion.
Fortunately, most comics have some elements of a multiverse in place, meaning that the explanation isn't all that hard to suss out. Allow some minor customization to a character's initial costume and you could easily get away with having each Superman come from a different parallel Earth, some almost identical to the "core" timeline and some completely out of left field. It's a minor issue that can be explained fairly easily, but it does need to happen.
You need a lot of choice.
Twenty-six characters is not a lot of choice. If you're selling your game on the strength of the player being the icon, you need to have a lot of icons. And that means focusing on having a lot of options for players who care about this sort of thing.
I freely admit that most people would want to be Superman, but I wouldn't. I would start diving into the more obscure members of the Legion of Super-Heroes or strange one-offs that have never gotten a lot of attention. Make Matter-Eater Lad or Huntress or Harbinger playable and we're getting somewhere. Over in the Marvel universe, I'd rather play Death's Head or Skids compared to Spider-man or the Thing, partly because of the audacity of letting me jump into game with a character that is not well-known but partly because these smaller characters are more interesting to me.
Seriously, I've read hundreds of stories about Batman. Playing as someone less well-known sounds more fun to me, both in terms of having more distinct abilities and in terms of just being unique.
This can cause design issues, but I think a clever team would find ways to work around them. A lot of characters in comics wind up falling into the same basic roles despite different powers -- Sunspot, the second Thunderbird, and Sunfire all wind up doing pretty much the same thing in practical terms. Using archetypes to define the limits of powers would certainly be an option, just as an idea. Either way, the mechanical concerns are something to be considered by the team angling for using pre-created characters.
You need customization
I mentioned before that you need an explanation for having a dozen Supermen in one place, but you could argue that Superman already comprises several different characters. Even without untangling the knot that is continuity, Superman in various stories serves as an omniscient father figure and protector, a powerful man struggling against even more powerful forces, an alien in human's clothing, a Kryptonian struggling to remember his planet's traditions, a devoted son, a friend who has to keep his friends at arm's length... the list goes on.
The trouble is that in some of those stories, Superman's actual abilities change as well. If he's going toe-to-toe with Darkseid, what matters most is that he's strong as anything. If he's trying to solve crises all over the city, his speed and flight are getting the big workout. He's the perfect protector; it's his invulnerability that saves the day. Depending on the story, his powers differ slightly.
It's not just Superman, either. Spider-man is stronger than a human being, but some stories really focus on his agility over his raw strength. At times his sense of danger is what keeps him alive; at other times he's mostly just clever and resourceful. There are different takes on how important any given power is based on the story at hand.
Batman? Sometimes he's the master detective, sometimes he's the master martial artist, sometimes he's the cruel taskmaster with a plan to defeat anyone. Cyclops is a great shot with his optic blast, but he's also a skilled leader and a resourceful fighter. Wolverine is able to be more than just a claw-happy berserker, sometimes acting as a detective or a soldier or even a guardian.
The point is that you need to have space for everyone to have a custom version of a given character because everyone's going to have a different picture of how this iconic character should play. And you're going to need to keep these characters playing differently based on the group they're in, most likely... but we're getting ahead of ourselves.
It's not that you can't do a game in which players are taking on the roles of iconic heroes. But doing it well, I think, might actually be more work than just letting people create their own characters.
As always, feedback is welcome in the comments below or via mail to firstname.lastname@example.org. Next time, I'm going to offer some useful tips for City of Heroes expats playing Champions Online as I continue to do so.
By day a mild-mannered reporter, Eliot Lefebvre spent years in City of Heroes before the world-shattering event that destroyed his home world. But he remains as intrepid as ever, traveling to other superheroic games and dispensing his unique brand of justice... or lack thereof.