1. I think the most important aspect is to develop your characters beyond one-dimensional cliches. Generally speaking, a few interesting characters will excite readers much more than many not-so-interesting characters would. Unless you’re doing children’s television, I’d recommend against a Power-Rangers-style setup where the members on a team have a single trait. For example, if your team consists of characters who have nothing going on besides a single trait/archetype (e.g. a hothead, a curious scientist, and an immature joker in Teenage Mutant Ninja Turtles), it’s probably less promising than it could be. In contrast, Tony Stark had all of those traits and I think it both made him a deeper and more interesting character while enhancing his dramatic possibilities with other characters (especially in Avengers). For example, Tony Stark’s curiosity combines with his lack of restraint when he decides to cattle-prod Bruce Banner to see if Banner has the Hulk situation under control. Batman’s preparation and paranoia come together in Justice League when he pulls out Kryptonite against a enemy and cryptically says he had it on hand as an “insurance policy.” In contrast, I think there are only two types of scenes between Raphael the hothead and Leonardo the hardass leader (scenes where they hate each other and scenes where they don’t). There’s only so many ways you can have characters act out a single trait with each other.
2. Another problem I’ve seen occasionally is where large superhero teams cut the roles too fine. I’ve seen 3-page synopses for stories which have (say) 8+ characters and half of the characters only get a line of description along the lines of “Avatar has fire powers and defends the base” or “Gridley is incredibly intelligent and is the team’s hacker” or whatever. I would recommend making your characters more versatile than that. For example, pretty much any superhero can defend the base–if base-defense is plot-relevant, just rotate that task among the notable characters or delegate it to a faceless extra that won’t take much space, but please don’t just randomly insert a character that will take space without actually getting to be interesting (or at least develop more interesting characters).
For example, let’s say a team has a scientist, a hacker, a soldier, an explosives expert, an outdoorsman/hunter, a negotiator, and a criminal. I think the most intuitive (though not necessarily best) approach would be to merge some of the characters (e.g. a scientist/hacker, a soldier with a background in wilderness recon and explosives, and a silver-tongued criminal). However, you can mix and match pretty much any of these archetypes into more promising combinations. For example, you could have a criminal scientist, a USAF hacker, a survivalist that knows far more about bombs than he can admit to, and a negotiator that enjoys coercion and/or blackmail far too much. Or a scientist that’s fascinated by explosions, a military hostage-negotiator (or a special forces operative with really good people skills), and a frightfully competent hunter/poacher who’s been coerced by the authorities into helping them catch the antagonist, etc. Hell, if you wanted to, you could probably combine most of all of those characters into 1-2 characters (e.g. a spy with both electronic and physical skills whose main job is tracking down a target and either convincing him to defect or eliminating him).
3. In most team stories (but not all), plot coherence comes either from a single main character (e.g. Mr. Incredible on The Incredibles) or from the team members spending most of their time together (e.g. Fantastic Four). If you have an ensemble story that isn’t about a coherent team (like Watchmen or Wild Cards), I’d recommend being especially sure to make the story feel coherent. For example, giving the characters common problems will help ensure coherence even if they aren’t constantly interacting. Come up with ways that their individual journeys affect other characters and/or create obstacles for each other. The first season of Heroes stands out there—in particular, Sylar (and secondarily the Company) created plot threads which made the story feel coherent than just 10+ superpowered people doing their own things.
4. With a larger cast of protagonists, I think satisfying protagonist-vs-protagonist conflict is necessary. Along with character development, I think that PVP conflict is the main aspect which separated great team and/or ensemble movies (e.g. Avengers and X-Men: First Class) from okay ones (X-Men 3 and probably Watchmen) and godawful ones (Batman & Robin and League of Extraordinary Gentlemen). In addition to three-dimensional personalities, I’d recommend coming up with ways in which each character’s goals and actions affect and/or set back other character’s goals and actions.
5. In a story centered on a lone superhero, you may be able to find space for a forgettable action scene or two (e.g. bank robberies featuring faceless mooks doing nothing more memorable than letting the hero show off his/her powers*). Ensemble pieces generally have less margin for error here—with so many characters pushing for development, it will be more obvious if time is wasted. Incorporating unusual actions/decisions is one way to use action scenes to develop characters — for example, Captain America asks a team of assailants preparing an ambush if anyone would like to back out before they get started, whereas someone like Wolverine would have attacked first while he had some element of surprise. Spider-Man breaks off a chase to save a civilian in danger, but the Punisher probably wouldn’t.
*In contrast, the bank robbery which opens The Dark Knight did an excellent job developing the villain and unfolded in a much more interesting way than a superhero coming in and showing off his superpowers on undeveloped bank robbers.
5.1. I would highly recommend checking out how X-Men: First Class incorporated both nonaction and action scenes into a central plot which at face value is mainly about action. I’d also recommend checking out how it incorporated dialogue into action. (Many less-effective stories focus on one-liners in action dialogue, but I thought that First Class’ action dialogue was both more substantial and more entertaining).
6. As more characters are added, there’s less time and space for each character. Among other things, this tends to limit how much space there is for aspects like the origin story and complicated superpowers. In a novel centered on a lone superhero, it probably wouldn’t be an issue if you took 10 or maybe 20 pages on the origin story. In contrast, if you have 5+ characters, there just isn’t that much space for each character’s origin. One workaround is a mass origin (e.g. in X-Men, Heroes, Static Shock and TMNT, the characters all develop their superpowers from the same source, although not necessarily in the same incident). In contrast, if you spend 5-10 pages describing one character’s demonic/magical training and 5-10 pages describing another character’s powersuit and 5-10 pages describing Captain Badass’ training and so on, that’s a substantial chunk of the book that probably isn’t contributing to the main plot as much as it could. (One possible workaround: perhaps the origins relate to the main plot in some way–maybe all of the protagonists were motivated or affected in some way by the central protagonist or antagonist or perhaps the origin stories affect each other in major ways).
6.1. Instead of introducing side-characters that mainly interact with only a particular character, I’d recommend focusing on side-characters that are more versatile. For example, TMNT’s Splinter can interact with all of the turtles, whereas Fantastic Four’s Alicia Masters is 99% limited to interacting with The Thing. If you have an ensemble of 5+ main cast members, it’s probably not very practical to give each of them multiple side-characters that don’t interact with other major characters.
