Game idea I had tonight, that feels like it's been floating in my subconscious for a long time now and only just come to the surface:
This is a combination of two different elements: Everyone's familiar with Pokemon, the idea is a 'capture and make'em fight' style of game.
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People familiar with Final Fantasy X will remember the story element where monsters in that game were supposed to be the souls of deceased humans that refused to move onto the next life, and so became monsters. In FFX this idea is honestly wasted, as it doesn't go anywhere and is just a bit of background lore.
My idea is that your main character is a "Summoner + Exorcist" in a fantasy world with a Buddhist-style cyclical death and rebirth system governing humans' lives, with a canonical physical afterlife-world that deceased human spirits are supposed to travel to in order to prepare for being reborn again in the living world. However, of course, there's a bad habit among bitter, resentful, and malevolent people's souls not wanting to travel to the afterlife and so linger on, taking on the form of monsters that are bereft of their original humanity, and have elemental affinities and abilities commensurate with the personality and abilities they had in life.
You'd have a mentor in the beginning of the game to explain all this to you, and your job as a Summoner is to find Spirits like this, beat them down, capture them, and take them with you on your journey, battling with them to get their anger out, and nurturing them to restore their humanity, and once they've leveled up enough, they take on their human form again and regain their sentience, they thank you and accede to being exorcised, leaving your party and going to the afterlife where they can continue the cycle of life, death, and rebirth.
As far as gameplay and game balance is concerned, as opposed to Pokemon or other RPG's where the designers make the safe assumption that you are linearly increasing in strength throughout the game and therefore all the challenges in the game are structured in a linear 'weakest first -> strongest last' that nearly all games conform to, the balance here is that the game is making the assumption that your Spirit Monsters are always going to be powering up, hitting their power cap, and then they are forced to leave your party, reducing your party's overall strength over and over and over down to a baseline level that you can only ever momentarily rise above, and so all the areas and challenges in the game are designed with this in mind; all the areas are started assuming you have nothing or almost-nothing, and end on the assumption that you have a fully powered up party that is appropriate to that area, before you get knocked back down in power in preparation for the next area; mirroring the idea of death and rebirth in the gameplay.
Of course, it would be aggravating if your Spirit Monsters were always just leaving randomly, especially before a boss, so gameplay-wise your Spirit Monsters would only leave upon beating a boss or other big challenge; where it would feel appropriate to say goodbye after a climactic moment of catharsis.
Earlier when I said "Take on their human form again", aesthetically speaking, the final 'evolution' for all Spirit Monsters is becoming a ghostly Jojo-style Stand user, with the monster that they 'were' being their Stand. The idea is that the humanity of the person they used to be in life has overcome the bestial hatred that consumed them to turn them into a monster, and now they have control over that bestial power in addition to their abilities they had as a human. They stay with you at this point, and only after defeating an area's boss do they reach full "Self-Actualization" to borrow an idea from Maslow's pyramid, and then they transcend to the afterlife; leaving you to venture to the next area and tackle that area on it's own terms, gaining new Spirit Monsters and repeating the process.
Pragmatically speaking, I'm thinking there'd have to be hundreds of generic human sprites or models, so while the Spirit Monsters themselves could be identical (as the corruption of hatred always reduces humans to a baser state), the resulting fully leveled up and actualized human that springs from it could be unique (or seem unique, at the very least) to give the player a memorable experience with every single one they train up. I'm thinking there could only be maybe 100 or so different voice lines or somesuch in addition to the human's models, the Spirits would have to be largely mute, leaving the player's imagination to fill in for what trauma had tied them to the world and what exonerated them enough to proceed to the afterlife.
As another game balance element, every Spirit Monster would have it's own 'lifespan', so there'd be some Spirit Monsters that have short lifespans that are very strong to start with, but they Self-Actualize very quickly and can only stay with you until the end of the area they are appropriate for; other Spirit Monsters would have long lifespans, starting very weak but they stay with you for a long time undergoing a large amount of growth and a number of different evolutions before they Self-Actualize and transcend. For example, the 'starter' that you get at the beginning would stay with you for either most or all of the game, itself being appropriate in strength for every challenge the game throws at you, and only Self-Actualizing and transcending at the game's penultimate climax.
Storyline-wise, I'm thinking there could be a sudden and bizarre uptick in conflict around the world, with wars breaking out and communities breaking apart, creating a global upsurge in these resentful Spirit Monsters' population and power. In addition, the religious group that is responsible for the recruitment and training of these "Summoner + Exorcists" (I haven't decided on an appropriate name) is itself suffering a spiritual crisis and schism in ideology that is itself creating more conflict and resulting in an all time low for the number of Exorcists that can handle this Era of Strife. This necessitates your character to make a worldwide pilgrimage to quell these Spirit Monsters and to try to set right a world that has gone wrong.
The finale of the game would have you venturing into the 'physical afterlife', similar to Dante traveling down into hell, but at this point all of your Spirit Monsters have been forced to Transcend and leave your party, so at this point you're traveling alone when up to this point you've always had company. As you travel down, you meet again the many rivals and villains that have instigated all the conflict in the world, and it seems like you're helpless against them, but one by one all the fully Actualized Spirits jump to your aide, as this is where they've all canonically gone when they've Transcended, and what seemed like a hopeless struggle at first becomes a heartfelt reunion with all the Spirits you've met and parted with along your journey joining you again for this final showdown.
I think that'd be really cool.