I liked Advent Children. It was a trip. Spirits Within has some great scenes, but overall wasn't great.

This is a long one!
Scroll all the way down to read the TLDR and gameplay details. If that sounds interesting I hope you enjoy the rest of the read. For those of you that want to dive right in, great! I hope you enjoy.

**Backstory

Starting from the bottom there is nowhere to go but up, right? Well, that's actually not the case. What you think is the bottom, is just the lowest you've been so far.
David knows that very well.
His parent's happiest day was when he was born. That was followed shortly by their most melancholy, when they realized they had to actually take care of him.

Very early he knew he was on his own. Digging change out of couches and raiding the cabinets for food while his parents were passed out was just a regular Tuesday for him.

He struggled, but he climbed. He first started working on small engine repair in the trailer park. His parents would always steal his savings if they could find them of course.
He got out of there as soon as he could, but a dark cloud followed. It rained and poured on him, but he was drenched a little less the further he climbed up out of his hole. A deep hole someone else seemed to have dug for him, and cast him into.

**Introductions

We meet David at his pinnacle. The seemingly endless ravine he's has to contend with is mostly behind him. He's a new father, and he knows he won't be like his parents, in fact he's determined to be everything they weren't. He starts his new job next week, and can't wait to get out of the metal foundry he's been working at.
Everything in front of him glows with promise and everything behind him seems further away, like a bad dream fading after you wake up... However, the true nightmare lies ahead.

A loud crack above him brings his focus into sharp clarity, and he looks up to see tipping scaffolding and a crane come off it's track. A casting ladle dumps it's deadly contents in a torrent of brilliant slag. There is no running. No escape. It splashes towards him in a path that, once again, seemed to be meant just for him.

**Descent

Shock and fear briefly compete before they decide to give utter darkness the stage.
His body is gone, and he is aware of this, but he's more surprised that he is aware at all. Phantom limbs grasp for something, anything at all, but no sensation returns.
What next? Wake up? He can't do that. But this can't be real. David tries to wake himself up. He thinks of everything he has to do and how much he wants to see his family. It's barely perceptible, but the darkness gives way just a little to deep shades of red. "I'm in a coma? I survived somehow" he thinks. The red deepens and he's back in nothingness.
He focuses his thoughts again, trying to recall everything he holds dear, somehow knowing he needs to do this, he needs to get back, he's going to climb out of this hole. It seems easier, like waking limbs, and he grasps hold of his will and narrows it like a spear. A bright red slit tears in his perception, and a rushing sensation of falling overcomes him.
He again pushes out to move his limbs and this time he gets a response, but completely foreign in feeling. A riggle, and a flail is felt, more than a controlled movement. The sensation feels wrong but he is more consumed by what he now sees, which can't be right.
He IS falling. What he sees is a limitless expanse of red and brown. A barren landscape beneath him, and rolling hills and mountains on all sides in that same reddish hue. When he turns to the right, just beyond the horizon, he sees a light in the distance. A break from the burnt rust color of what was either the sky, or the walls in some incomprehensibly large cavern.

As the light shrank over the horizon his gaze shifted to the rapidly approaching ground, which he soon hit with a wet thud.
Consciousness seemed like it would fade again and he would awake from this hellish vision, but it all came back into semi clear focus.

**Welcome home

David tried to right himself to take in his surroundings and was again met with a twitching, lurching sensation that didn't resemble limbs. With great effort, and a little concentration, he doubled over to look down at his new shell. A long cylindrical worn like form. It was impossible to gauge size in this strange place with these strange new features, but before he could begin to speculate he felt a nudge at his side which toppled him over.
Horror gripped him as he shifted perspective and saw that the ground wasn't the ground, but was just more worms, just like him. It writhed lately as this realization sunk in, and he reeled back as quickly as he could.
Most of the worms were featureless. Miserable meandering things that bumped into each other and shuddered. He could see some of them had a hint of an eye, or a slit where he imagined where his eye would be. Others seemed to be completely blind, and just had the vague suggestion of mouth.
He noticed something else. He had a slight feeling of sinking and sliding, and now he could see that this was because the others were moving away from him.
It was then that he realized it was easier to move. Maybe he was getting used to these new strange sensations, but it also seemed that his new form behaved more predictably.
He could see all of these things because apart from the faint light that seemed to permeate everywhere, he himself emitted a light that shown out around him. And, there were others. Off in the distance he saw illuminations he couldn't quite make out, but seemed to be slowly moving about.

New sensations started to press in. Almost like physical sensations, he could feel the presence of these other roaming lights. Somehow he knew there was one far behind him, traveling from his left side to his right, so he turns around to see it, and doesn't feel surprised at the verification of this fact.
Suddenly, David notices most of the worm around him stop moving, as the sensation of another presence grows near. Worms fly in all directions as a larger worm bursts from under the pile, straight for him. The creature produces a mouth. Misshapen and garish, it snarls open to reveal decaying yellow teeth. It bites down on his lower section, drawing various fluids and tearing into his new form.
Pain isn't the right word to describe what David felt. He was suddenly aware of a weakness upon getting bitten. His grasp on this warped reality faltered, and the area around him dimmed. Not because he was losing consciousness, but something else.
The creature took another bite, and seemed to restore the luminosity of the area in doing so.
This nightmarish scene pressed heavy into David's mind, and he thought he could maybe go to sleep now. Maybe it would be the end of the nightmare, or just the end in general. It didn't matter. The oppressive disparity seemed to sap him of all need to care.
He started to shut everything out and became aware of a lower state. He could now see the light from the worms around him. He could not only see them, but he could hear something. Whispering. Faint and distant, but an endless chorus of words. They started to separate and become more distinct. He vaguely become aware of being bitten again on the other end of consciousness as the words materialized.

"This is it. This is all that's left for us. Why are you fighting it? You think you're special? Different? Better than the rest of us? Well this is the end. We are all here. Anyone you knew who passed is here. We can tell you don't believe us. Well, do you want to talk to your father? Oh, you didn't know he passed? Well then..."
After a pause a familiar voice filled his mind.
"Davey, I know I messed up, but it doesn't make a difference here. We are all here. The best and the worst."
David recoiled at this. Hearing a voice he hoped he never would. Losing everything right when he had come so far. Knowing he didn't belong here. Somehow knowing as long as he had his own will, he wasn't dead. He wasn't done.

**Rebirth

Like a bursting dam, everything he ever wanted to fight for resolved in a force of will and the red world opened back up to him. Rushing clarity burst into his perception and he saw his attacker impaled on a bony protrusion. An extension from his own body. The dead worm shriveled in this calcified spear, and seemed to melt from it.
An internal swelling filled David's new form, and he reached for his old limbs and felt resolution at the tip of his senses, as a mockery of a the human form burst from his pupa like form. It lay in sheets below him as he slowly stood up, gaining old sensations back that he didn't realize had already grown so distant.
The fleshy, undulating ground held firm under him, as he felt the worms turn ridgid with fear at his new presence. The brightness increased around him, tenfold as he pushed his new senses outward to survey the world anew. Not as one trapped in this horrid place, but as one determined to climb his way out.

**Full Story Summary

The story will follow David as he familiarizes himself with his new existence. In order to progress in this deadly, alien landscape he has to resort to first taking down larger creatures with his cunning, as well as using his own willpower as a weapon to change his form.
Taking down other more advanced creatures he learns to integrate their pieces and components into himself in order to be adequately suited to dominate his would be foes.
As David progresses from smaller creatures to larger ones he starts to learn things from merging with these beasts. There are many others like him that want out, and take on new forms to attempt this. However there are others, who feel perfectly at home here, and by the force of their will they create armies to hunt others like him.
His progression leads him dangerously close to abandoning his humanity at several junctures. He also finds out more about the relation between human consciousness and this strange dimension, as he makes allies that have spent a great deal of time existing on this plane and learned a great deal.
The progression to the top is gradual, but eventually David makes it to the light he saw in his descent. The light is a giant lense that lets in brightness from the other side.
Based on the decisions of David to dominate or attempt to heal this world, the lense will show him alternative endings and he will either learn to let go of his mortal life or breach the lense to bring his hordes to earth to rage against the unfairness that inevitably led all paths to this conclusion.

**Gameplay ideas

I think this would best be adapted to an RPG twin stick shooter/slasher. There would be puzzles in order to beat enemies and upgrade sacrifices that would force the player to take certain paths that suit their gameplay.
There would be frequent encounters where the player must learn to either take a chance on trust to gain allies, play aggressively and take what they can, or attempt a more covert combat style to sneak their way to the end.
Three combat styles would follow each choice. If you go with allies you'll be giving commands and summoning in order to effect your attacks.
Playing the tank you treat your body like a vehicle. Taking down whatever unique creatures in order to rip limbs, weapons, and other features off to add them to your own build.
If you play stealth you focus on maximum damage and camouflage. You can still have allies, but the number is limited.
The player will guide David through the hierarchy of bosses and look for shortcuts in order to deal death swiftly to an individual boss, or skip him entirely (which then means they give up the opportunity to get a specific upgrade)

**TLDR

David has lived a brutally difficult life. After a great deal of time and effort he's able to crawl out from under it... Almost. He dies in a work accident and find himself a part of a strange corpse of a world, with strange rules and limitless dangers. It's here where he realizes that he's not dead, just displaced, and decides he has to use his new powers and strange new body to find a way out of this and back to his life.