In which I am a weeb

So, I've been watching Made in Abyss recently. As anime goes, it's one of the best I've seen, and incredibly OSR.

Here's some screenshots.

For those who haven't seen it, the plot can be summarized as this: a giant chasm exists with various progressively weirder and more dangerous environments the further down you go. It's also full of valuable treasure. A society exists of 'cave raiders' who live at the entrance, and go on missions into the abyss to explore it and bring back treasure. These cave raiders are divided into ranks (color-coded by the type of whistle they carry), with more experienced ranks being able to venture deeper into the abyss: the highest ranked 'white whistles' are all unique adventurer-celebrities, and hinted at being not properly human anymore. Oh, and while going down the abyss is easy, returning to the surface makes you sick, drives you mad, mutates you or even kills you. This is known as the 'curse of the abyss', and it's a genius bit of worldbuilding; after you've gone past a certain point, the only way is down.

Anyway, an orphan living at the abyss's entrance finds in quick succession a robot boy and a message from her mother saying that mum's waiting for her at the abysses's bottom. So, off her and her robot pal go on a quest to the abyss's bottom: ADVENTURE ensues, but is swiftly replaced by HORROR and PAIN.

So, why am I talking about this? Well, firstly, if you like oldschool D&D, go watch this show, as it's packed full of interesting ideas that you can steal or adapt. Even just the 'you can go down, but you can't come back up again without pain' mechanism could make for very interesting adventures.
But also, it's got me thinking about game design. I genuinely think you could make a good OSR game set in this show's world.
How I'd do it:

• Level is equivalent to rank. The show's fiction lays this out pretty clearly, you start out as a red whistle, and if you survive and succeed at bringing back relics, you progress to blue, moon, black and finally white whistle ranks. This sets up a nice simple 'xp for relics' loop.
• Each level you gain is a dramatic jump in power (after all, there's only six ranks). Red and blue whistle characters seem mostly mundane, whilst white whistle characters are depicted as monsters. In particular, I think that when you hit black whistle you have a few mild 'transhumanist' options, whilst at white you get to pick from a lot more. This makes high-level PCs all feel unique and bizarre compared to the more grounded low-rank PCs.
• Different levels of the abyss are pretty easy to model with random encounter/environment/treasure tables. Each has a few basic elements when you arrive there. Going up or down will eventually bring you to a different level. Going sideways brings you to a new area, where you roll up a random environment, and maybe encounters and treasure. To return to town safely, you'll need to backtrack the way you came. As you explore, you get a branching network of areas that you've already discovered. Possibly the more 'steps' down this network an encounter is, the better the rewards are, encouraging you to press further away from easy retreat.
• The Curse of the Abyss is pretty easy to model. In the first layer, every ten feet you go up deals 1 damage. At lower layers, you might get things like attribute damage, save-or-suck effects, save-or-die effects and so on every time you go up ten feet. 
• Monsters in this show are rarely defeated in straight combat (reg the robot's giant lazers not withstanding) and instead must be driven off or escaped from. Give them loads of HP, but make damage to them force a morale check. Make 'flee' an action in combat which probably succeeds (although the monster gets a free attack at you as you go, maybe) but dumps you in a new random area.
• As well as a core book explaining the base settings and rules, each level has it's own mini booklet, explaining new rules for stuff you can do now you've levelled up, and giving you IC information about the world. As you level up, more and more secrets open up to you.

I'm working on Esoteric Enterprises as a project right now, which is likely to be a while (several months) until it's done. But this is something I'm tempted to put on the backburner and start brainstorming ideas for: I think it's got real potential.

Seriously, watch the show, it's good.