I was thinking this morning, outside in the frost. Aside from re-visiting the organs and their functions, here are new, interesting things.
First, the sanity system. The sanity system would be based on how much work the organism is doing, and how much brainpower it has. Doing a single thing, monotonously, would decrease the sanity of independent organisms (IE, if the organism has > X independence, it will get bored and frustrated). Doing a lot of different tasks would overwhelm a dependent organism, unless it's smart enough. Doing complex tasks (IE, crafting something that requires crafting materials that need to be crafted from things that need to be crafted, etc.) would overwhelm a low-brainpower organism. Doing nothing at all would bore a sentient organism. Etc.
Before I go in what sanity does, I need to introduce Disposition. Which is to say, what the organism feels about you.
For every consumable item, a random number would be assigned, from -1 to 1. It would be on a bell-curve, with around 0 being the most common, and the extremes would be equally uncommon.
The more sentient and independent the organism is, the more of a personality it has; the more it likes and dislikes. Feeding it things it likes will make it like you, and feeding it things it dislikes will make it dislike you. However, what it wants at any given point is in flux; today it may want chicken, while tomorrow it may want ink sanks, and the day after it may find wheat to be disgusting.
It will also like it when you communicate to it, and actually ask it what it wants to eat or do, or if you ask it what it wants (and then doing it). It may also ask things of you, such as "Could we make X" or "We require more minerals, could you be a dear and grab some?". These demands, and how much they mean to the organism, would be random, yet based on its independence, brainpower and sentience. The smarter it is, the more utilitarian it is. The more independent it is, the more likely it is to actually communicate its desires. The more sentient it is, the more it cares about its own desires (And a super-low sentience would mean that it didn't have any desires).
Now, then, sanity would play with disposition and communication. If the creature dislikes you, or fears you, it may lash out and attack you. If it likes you, it may close you in, try to eat you, or try to hug you violently. Going insane might also change the disposition wildly; it may love you one minute, then demand you get out of its house. And then it may start to miss you, and start to eat itself. Or maybe it'll suddenly desire world domination, making lots of minions. Or it may wish to eat the world, expanding and growing until there is nothing but the organism. (Galacticraft would be nice here; the creature may desire to become the entirety of the universe)
Of course, if your organism goes mad, there's little you can do. Indeed, there's only two solutions: To flee, or to dismantle its brain and try again.
------------------------------
I also thought of a new mob, and a new organ (with a new system that comes with it).
The Observer: This would not be something you crafted, but something that the organism creates. With 8 oculuses (oculi? Whatever), 8 spider eyes, 8 ender pearls, 8 tentacles and 64 feathers, this would be a floating, tentacled spider-creature that simply.. watches. If attacked, it will teleport away, and it doesn't need to be physically connected to the organism to send and receive information.
Abominative Chamber: This may be a tad disgusting, possibly sick, but extremely useful. First, take a tentacle and 5 iron bars (not ingots, bars) to make an abduction tentacle. Then use said tentacle to capture something- a spider, a villager, a sheep, whatever- and stick it in the Abominative Chamber. Eventually, the creature will become an Abomination; a mindless creature whose life is nothing but misery and agony. A spider abomination will grow an excessive number of eyes, and pustules full of string grow in and outside of its body. Shears are then taken to it, the eyes and string harvested, leaving the abomination barely alive. But as long as its alive, it will continue to grow, and thus continue to be harvestable. You should probably have the abominations sheared regularly.
Human abominations will grow large tumours of flesh and bone. Harvest their flesh, and ignore the look they must assuredly must give you.
Chickens will develop large sacks of eggs that grow from their nether regions, their feathers grow quickly and fall off their constantly-expanding bodies.
Cows may possibly have it worst, though. Their blood changes to milk, their skin sheds, and they will constantly birth sacks of flesh.
Pigs simply lie in their filth, their bodies expanding and expanding, never stopping. Their bodies will not be able to support this growth for long, but that doesn't mean that they will be allowed to die.
Sheep have it easy, they just grow extra wool. A lot of wool, but it's still just extra fluff.
Creepers will constantly fill up with gunpowder, which means that they need to be cut up to let it pour out. You wouldn't want one to burst, would you?
Lastly, endermen. These are tricky to contain, and thus need a special Abominative Chamber. They will not be harvested by the player, but instead require an intelligent Organism to manage.
They will be mostly liquefied, leaving them as floating gristle in a vat of themselves; the water will hurt them, and they will try to teleport away, but the liquefied ender will instead pour out of their mangled, barely-solid bodies. The amount of liquefied ender inside the vat must never exceed half of the amount of water, for otherwise they will escape their constant torment (and you will stop getting near-free ender).
---------------------
While I don't fancy adding worldgen, I think there is one thing that this must have: Hostile organisms, and infested towns.
To show the player that what they are doing is exceedingly dangerous, towns could spawn in the world that are slightly.. off. The villagers won't leave their homes, zombies won't spawn, the water burns, and the crops are disgusting. Should the player dig under the houses, she will be confronted by hordes of tentacles and infectees, while the villagers become hostile and swarm you. The earth beneath the town has been artificially hollowed out, the walls covered in pulsating flesh. Gaping mouths burst from the floor and the walls, attempting to drag the player away from the center, where the addled brain of a monster silently squats.
Should the player slay the brain.. she is not saved. The tentacles, the minions, the villagers, all now go mad and attack each other as well as the player; the chamber starts to crumble, collapsing in on itself.
..well, maybe that's too much to code, but you get the idea.
