I've been thinking more about that Fire-Emblem-ish game idea I had. So I've written a bunch of thoughts below.
I mentioned in the tags that it would work better as a superhero-genre thing. A premise like "you can recruit anyone" kinda requires a small cast size, and superhero teams tend to be smaller than fantasy armies.
Also, superhero narratives often feature fluid allegiances. Superheroes become villains and vice versa, heroes fight against each other before teaming up again two issues later, antiheroes shoot whoever the writers say he has beef with this week.
One other difference between superhero media and fantasy war epics is the death toll. "Low fantasy" like Fire Emblem or A Song of Ice and Fire tends to be pretty lethal; often, they'll kill off characters just to prove they can. By contrast, superheroes usually only die in dramatic moments, and they usually don't stay dead.
Anyways, the point I'm getting at is that I want this game to have a way to "rout the enemy" without necessarily killing anyone. Character death should be a deliberate choice or a result of at least mildly unusual circumstances, not something that happens every level.
(That also means I can keep the cast relatively small, without needing to include several new antagonists every chapter to replace the ones who just got killed off.)
So I think characters should have both Heath (normal hit points) and Morale. Attacks and anything else that affects Health would also reduce Morale, balanced so that Morale generally runs out first, and some effects also reduce Morale. Killing a character should damage the Morale of all their allies, for instance, and powerful characters running out of Morale does the same.
A character who runs out of Health dies, but when they run out of Morale, they start running away instead. Player-controlled characters who retreat like this will be available in the next mission, by default.
Modern Fire Emblem games usually have defeated characters return, but with an optional "ironman mode" where they instead die permanently. This game would have a "stark mode" where player-controlled characters who retreat (aside from the player character) essentially un-recruit themselves. They're not dead, but they aren't on your team any more.
Stark is, of course, a perfectly common adjective and not a copyrighted character's name.
As mentioned, Morale usually runs out before Health. Ways to kill a character include:
Dealing damage to a character who's already out of Morale. (either catching them in some kind of area effect or just shooting them as they run away)
Attacking someone with high Morale. (ie, someone who refuses to give up until they are physically incapable of fighting)
Repeatedly attacking someone whose Morale, but not Health, is being replenished. (ie, someone kept in the fight longer than they can withstand)
Attacks that deal a lot of damage in one shot; either enough to flatten a character in one shot, or a less OP attack applied to a target who's alreadu on their last legs.
Ideally, I want characters to die either through deliberate action or recklessness. You don't have to kill anyone you fight, but you can, and killing important characters should have story consequences.
...which does complicate the story structure of this hypothetical game. Characters can be recruited, or unrecruited, or dead. I don't think I'd need to make a distinction between "never recruited" and "recruited then ran away in Stark Mode" or whatever, but there would absolutely be situations where the distinction between "you killed Exampleman" and "Exampleman died fighting for you" is important.
The more I think about this, the more I see pressures to keep the cast small. A single-digit number of central characters, plus a bunch of characters that might have interesting subplots but won't drive the plot by themselves.
One other thing that makes me want to keep the cast small: Superpowers.
In Fire Emblem, everyone's powers fall into the categories of "big metal stick," "small long-range sticks," "magic sticks," or "magic books". Except the ones who turn into animals, but they're functionally a big metal stick with extra steps.
But superheroes all have distinctive abilities; that's kind of their whole deal. I'm sure you could fold all of those abilities into a handful of mechanically-homogeneous categories. But if the cast needs to be kept small for narrative scope reasons, we might as well give each of our small number of characters unique superpowers.
You know how, in a Fire Emblem game, you move and then take an action? Usually Attack or Wait, sometimes Item or Dance or Rally or Staff; occasionally something situational like Talk or Convoy, or unique to that game like Pair Up or Capture.
Anyways, in this hypothetical game I'm describing, one of the common actions would be Power. And it activates a superpower, unique to each character. Some are flashy attacks, or healing, or other support abilities. Probably some characters have situational powers that usually aren't helpful, but are essential or handy for certain chapters.
There would probably be some overlap between different characters' powers, of course. Any character whose power is mostly a way to blow things up at a distance is probably going to have some kind of special attack as their Power action. But hopefully they feel different to use.
There would probably also be some unpowered characters. They'd fill out the rosters on both sides; extra cannon fodder for antagonists (because a Fire-Emblem-style map with half a dozen enemies sounds like it'd either be boring or hard to design), and a way for the player to sort of keep up if they failed to recruit many units and/or had a lot of casualties.
Brawl. Physical melee. High damage and toughness, but most targets have better physical defense than energy, and their powers tend to be less impressive.
Bolster. Physical ranged. Powers which are not directly useful for combat, but which can support others. For their neutral special, they wield a gun. Good powers, weak stats.
Burst. Energy ranged. Lasers and stuff. Powers tend to be AoE, extra-long-range, or otherwise special attacks. Tend to have balanced (but mediocre) defenses. Also tend to have secondary movement powers (e.g. flight)
Bloom. Energy melee. Instead of being big and strong, they physically transform in some way; turning into a ghost, or setting themselves on fire, or something along those lines. (Breaker-type parahumans.) The only basic class which usually has a better energy defense than physical.
This class system probably doesn't need "promotions"; it could function on its own like this. Class defines attack types, and maybe some other mechanics interact with class in some way. Like, something that forces enemies to attack you, that only works for brawlers.
But if there were "promotions," each character would have a different tree of class promotions, which would probably come with various types of power upgrades.
Unpowered characters would, of course, have different classes. Probably less impressive ones. I think it would be fun if unpowered characters had a more complex, rigorous class tree...but were still less effective overall than characters with powers. Being really really good with a gun is no match for being able to throw cars with your mind.
The tutorial mission features the main character and several other prominent superheroes fighting some kind of mad scientist villain, who has created a bunch of interchangeable goons. Robots or mutants or superintelligent gorillas or somesuch.
Once the player concedes to the game's suggestion that you try recruiting enemies, the villain freaks out at the possibility of his army turning on him and starts blowing up any that you try talking to. It doesn't take many casualties for the goons to turn on their maker. This keeps the promise of recruiting any character (you can recruit any single goon), but prevents potential problems from players being able to recruit a large number of interchangeable tutorial enemies. (Or filling the first level with a complicated cast of characters before you have a chance to meet any of them.)
Possibly the goons are some kind of genetically-engineered ape? More original superhero universes should include an uplifted primate who either solves or commits crimes among their roster of Silver Age archetypes.
Anyhow, this mission would of course serve to introduce the player to many of the game's general mechanics and peculiar quirks (hence the emphasis on recruiting one of the goons). But it also establishes that, at one point, the status quo resembled a classic superhero universe. All the heroes cooperate in fighting an unambiguous supervillain trying to take over the WORLD!
After that, there's a time skip to shortly before the superhero "civil war". One or two missions with the heroes all nominally on the same side, but the cracks are already starting to show.
Establish what the status quo is after everyone's been doing this for a decade, after the superhero thing turned from a side hustle or hobby into a world-shaking institution. The conflicts both within the group, and with external groups, and how the latter fuel the former.
And then you're forced to pick a side and go down one of the main "routes".
Marvel's (in)famous Civil War event has a solid premise for any story about superhero-on-superhero conflict. The government wants to regulate superheroes, and the superheroes are split between those who want to accept this oversight and those who want to reject government interference.
It's straightforward, it's a conflict superheroes would face if they existed IRL, and it cuts to the heart of what superheroes are. The side chosen by each superhero is necessarily a result of how that superhero understands their superheroic role. Are they cops with superpowers, or free agents enforcing their own justice? Either option is fine if everyone involved is a Good Guy, but either one could quickly collapse into dystopia if a bad or even ordinary person was involved. If there are no good options, which is the lesser evil?
I'm imagining that both sides have 2-3 main superheroes, who are united by the immediate pressure of the "registration act" with strikingly different motivations. Everyone on the "pro-reg" side supports the "registration act," but for different reasons; perhaps one has a philosophical belief that superpowers should be controlled by the masses through democratic institutions, and one is afraid to take sole responsibility for how her power is used (as the "anti-reg" side expects), and one is just super patriotic.
These different reasons can cause the two factions to split in many different directions as plot pressure demands. A faction which starts to win might splinter, as the once-minor disagreements over details become more relevant. Or an opportunist might approach a superhero with compatible beliefs and try to win them away from their faction.
The main character is a telepath. This telepathy is the justification for some game mechanics (like being able to see exactly how much health and morale enemies have left), but it also means the main character's power is long-distance communication. (Possibly with options not available to other characters who try talking.)
The main character's power would also be good for manipulating morale. Either unnerving them for extra Morale damage, or bolstering your allies' courage (at the risk of making them run out of Health before Morale).
Personality-wise, I think the main character should just be a general do-gooder. They want to do whatever's going to help the most people, both for ethical reasons and maybe because of their powers. Someone who can feel the background misery of whatever city they're in is going to be invested in improving the lives of everyone around her!
This puts them in an awkward spot. Everyone knows that a conflict among superheroes (or between superheroes and the government/military) would be devastating, and most expect the world to be worse when the dust settles. But by the start of the (post-tutorial) story, pretty much everyone is resigned to conflict breaking out sooner or later, with the protagonist being one of the only prominent superheroes trying to actively prevent it.
This characterization unmoors the protagonist from the conflict. They have, after all, actively avoided taking a side; this leaves them open to choosing either side, as the player desires, while staying in line with their previous characterization. (It also lets them change their mind, if the script-scope allows it.) And it means that their perspective on their own role as a superhero is underdeveloped; they haven't put that much thought into what it means to be a superhero, because they're busy maintaining a world where that ambiguity is acceptable. This leaves them open for character arcs in several directions.
It also gives characters a reason to treat them as a "third option". Characters who don't want to choose between the "pro-reg" and "anti-reg" factions can instead side with the "let's not fight" faction. This gives the player character a role with some agency; not just a significant underling to one side or the other, but a third party which has allied itself to one side in the interests of ending the war quickly.
This explains why you the player doesn't lose half of your recruits when your faction splinters; they're your recruits, loyal to you, and they whichever faction you choose to align yourself with. It also puts the player character in a position to call shots, which plays into the appeal of making decisions that impact the narrative.
