Anyway: the idea is you play some kind of generic monster/'thing' and do monstrous stuff instead of being the stereotypical heroic adventurer.
STATS & RESOURCES
Stats
Strength: Affects melee and sometimes ranged damage, carrying capacity, some activated melee talents.
Dexterity: Helps with attacking and defending, some combat talents.
Constitution: Used to determine Max Life and Stamina, and for many monstrous abilities.
Resources
Life
Stamina
(eventually)Rage/Hate/Blood/Flesh Eaten
Other Counters/Variables
Size
Reputation: How known(and feared) you are, related to random quests/enemy NPCs being generated
DESCRIPTORS
You get to build your own monstrous creature. How do you do that? By picking from many possible choices.
Here is what is in/planned currently...
Origin
You are a monster, but how did you came to be?
Natural: The most mundane monster type - you might be a beastman, dragon, ogre, or even just an unique beast...
Other natural creatures such as animals are much less likey to be hostile, and you might be slightly better at some nature-type talent trees.
Arcane/Eldritch: You were turned into what you are by sorcery, or perhaps are a sorcerous/extradimensional being yourself, summoned to this plane. Examples would include Demons and Lovecraftian Horrors.
You will get a bonus to sorcery-type talents/skills, and maybe mana regen.
Primordial: You are more than natural, the embodiment of some powerful aspect of nature or even of the universe itself. You might be a rampaging rock elemental, but also an avatar of Death...
You will have mastery bonuses to the elemental talent trees/sorceries, in addition to some very specific bodytypes and elemental resistances/vulnerabilities.
Human: Humans get no monstrous talents(and are naturally blimnd in the dark...), but get a bit more skills, and have starting equipment(not very impressive one).
Bodytype
Suggestions for other names accepted. Basically, your rough physical structure.
Humanoid: Roughly human-shaped, you can use all equipment types.
Bestial: Still somewhat manlike in form, but not enough to use all kind of tools(limited equipment selection basically). However, you might have a mastery bonus to physical talents.
Shapeless: You are some kind of shapeless blob of ...stuff that, while fairly flexible, cannot use any equipment meant for humans. However you are very tough and hard to kill, and regenerate very fast.
Elemental: Includes several special options, such as being made of rock, flowing water, fire, air..
Horror: You have far too many tentacles, maws, antennaes. Can use some equipment types(weapons/stuff held in your hand), has some 'horror'type powers.
Quadruped
Serpentine
Size
Fairly simple.
There are three starting sizes:
Small(-STR & CON, +DEX, size 4), Man-sized/Average(size 5), and Large(+STR&CON, -DEX, size 6).
Note that later in the game, you will be able to become larger with the 'Growth'talent. Thus even if the monster you have in mind would be best described as 'Huge'...don't be worried, as you'll just have to grow a bit

Theme/Focus
Not sure on what this should be but... but basically a special 'something' your monster has, does, or wants to do. Also related to winning condition/score, if there will be.
Death: Necromancy like stuff, powers, talents.
Greed: You are interested in hoarding stuff, like the stereotypical dragons.
Slaughter: You enjoy mercilessly slaughtering other beings, both monstrous and human. +Mastery to combat talent trees.
Humanity: You want to interact with humans, +mastery to the 'social'talent trees. Could perhaps be renamed to something like 'Trickster'.
Conquest: You want to conquer as much as you can, ruling over both humans and over monsters. There might also be a 'summoning' aspect to this, handled as +mastery to the spawning talent tree.
Perks
Not necessarily monstrous, but lets you personalize your character even further.
None: Picking this lets you have no flaw.
Charming: Chance of human/adventurer NPCs not being immediately hostile?
Cunning: 10% less exp required to level up
Gifted: +1 to all stats, +1 talent(if monster) or skill point(if human). Perhaps also gain extra talents/skills every X levels.
Quick: +10% global speed
Lucky: Higher chance to get positive events
Relentless: +8 CON, +0.5 life regen(Yes. it's powerful, easy mode basically. Meant for the monsters that are truly hard to put down.

Spawn of Chaos: +4 CON, start with a random monstrous talent AND gain another at every level divisible by 5. For those who like surprises

Flaws
As with perks. Note that flaw selection is limited by your perk - if you picked Cunning, you can't be Dumb!
None: Only available if you picked no perk.
Cursed/Unlucky: Higher chance to get bad events.
Darkling/Nocturnal/Creature of the Night: Vulnerable to light and fire based attacks, loses stamina/speed in lighted squares
Horrifying/Reviled: More human/adventurer NPCs attack immediately, more hostile monsters
Dumb: 20% more exp required to level up
Sluggish: -10% global speed
Mundane/Underdeveloped/Immature(pick a name): Two monstrous talents less.
Other descriptor types I'm considering(but about which I'm not sure, don't want to overdo it):
habitat: swamp, underground, desert, forest, water....
background: this is what you actually did before the game started, how you lived your life, would affect starting dungeon, skills, lots of things. If you lived with other monsters(or humans) you could have been a hunter, farmer, craftsperson, warrior...
Also, instead of creating a custom monster, you could go through character creation faster by using some templates. These would be generic options which would force some(though not all descriptors)
Examples could be:
Ogre:Natural, Humanoid, Large
Imp: Arcane, Humanoid(or something like Humanoid with wings+tail), Small.
Dragon: Natural, Quadruped(or Serpentine?), Large, Greed
This will also be useful for generating NPCs/monster races - we don't have to adapt existing monster types, we can also create new ones.
TALENTS
There are two kind of talents.
SKILLS, which deal with using tools, items, fighting and doing stuff that humans/non monsters could also do(although humans could do it better) - like most talents from ToMe, though a bit more generic.
Some skills/skill trees:
Melee Combat: Might not bother with sub-skills for each weapon type. This gives a + bonus to-hit, and a +% bonus to damage.
Ranged Combat: Obvious. One idea was that the more advanced/human weapons had a skill requirement, so for example slings only took 1 level in this skill to use, bows took 2, crossbows took 3(or 4)...
Armor Use: Gives a +% bonus to protection from equipped armor.
Shield Use: Gives a +% bonus to defense from equipped shield, maybe a small chance of getting a counterattack if you block.
Stealth: Quite similar to the ToME implementation.
Traps: Possibly deals with both setting and detecting them.
Defense(better name?): Basically, gives a better chance to dodge attacks.
Healing: Nonmagical(mostly), works with herbs/mushroom/medicine items
Human Lore: Helps interacting and fighting with humans.
Monster Lore: Helps interacting and fighting with other monsters.
Nature Lore: Helps interacting and fighting with non-monstrous natural beings(animals, mostly).
Sorcery: Not going to have as much magic as ToME, but there will be some spells.
and MONSTROUS ABILITIES(again, need a better name) which only monsters get(...usually)
Here are some monstrous talent trees.
MONSTROUS PROWESS(generic boring-but-useful monstrous stuff)
Natural Armour: Should be self-explanatory. +Armor, maybe - to enemy crits.
Natural Weapons: Could be split into several lines, such as tail slap/gore/claws, some could also give access to active abilities rather than just +combat damage.
Health: +Max Life and life regen.
Endurance: Does to Stamina what Health does to Life. Should be useful, as many activated/sustained powers will consume significant amounts of stamina.
(one could argue that Health and Endurance could also be skills...maybe I should have lesser versions that are non-monstrous, yes)
Growth: Each level(there will be less than 5, probably 3) makes you bigger, giving +STR, CON, size, but -DEX.
HUNTING/STEALTH
Note that 'basic' Stealth, as in the ToMe talent, is going to be a skill, and not a monstrous talent.
Sight: +vision range, infravision, see invisible. Note that default monsters would already have some see-in-the-darkness capability.
Scent of the Predator: Lets you detect other beings, should not work all of the time and certainly not with all creatures.
Lurk: Gives you a stealth bonus/invisibility as long as you stay still or move slowly.
Lure: Draw NPCs to you, even if they are fleeing or not interested in fighting you(this could arguably be in GLAMOUR, though).
Sprint: Significant speed bonus at a huge stamina cost.
GLAMOUR
Note that these powers will have a reduced effect on other monsters, they are mostly for use on human-type NPCs. Could be worth it to have a social stat just for this kind of power.
Glamour: Confuse/stun NPCs.
Skinchanging: Change your appareance/faction to avoid being attacked. Perhaps requires eating corpses first.
Charm: Change hostile NPCs to neutral(and *perhaps*allies?) for a short time.
Dread: You fill the heart of your opponents with fear. Sustained talent, gives penalties to opponents around you, some of which might just turn and flee.
RAMPAGING(better name, maybe? basically stuff that stereotypical huge monsters would do)
Like Devouring, this is a talent tree for large monsters - most of the powers here are affected by size(and Strength)
Roar: Stuns/confuses creatures around you, does some light damage.
Onslaught: Similar to the ToMe version, strike multiple opponents at a time.
Quake/Tremor: Strike the earth, stun people around you for a while.
Juggernaut: Sustained ability, take a penalty to speed and defense, but gain +armor and +knockback with each attack.
Hurl: Grab a monster smaller than you, throw it at another monster(or against a terrain feature).
Rip: Attack a terrain feature, such as a tree or boulder, and turn it into a weapon(which can also be thrown)
DEVOURING
Devour: Similar to the 'Swallow' Wyrmics have in ToMe, but requires the target to be smaller than you(thus synergizes well with Growth).
Spew: You can spew some of your stomach acid, dealing significant damage over a small area. Could perhaps consume a 'fullness'resource instead of stamina(you'd get fullness from devour).
Burrow: Your jaws and metabolism are now strong enough to chew through stone.
Cast Iron Stomach: Lets you eat equipment to regain a bit of Life. Depending on your level in this talent, you can eat different materials. Eating higher-grade/magical materials might give temporary bonuses to stats - for example eating a ring of protection could give +defense/armor for a while.
SPAWNING
Summons...sort of.
Basically this tree will have talents that spawn weaker monsters that will fight at your side for a while. Would be interesting if their stats and talents were based on yours - they ARE your 'children' after all.
Could also include poison/acid/blight/disease resistance, as this talent tree would be somewhat related to swamps and fertility.
I might also have mutually exclusive talents trees(say, elemental related ones).
EQUIPMENT/ITEMS
Not sure about this.
One idea is that using 'proper' civilized equipment will be optional, and will require special talents to get the most out of it.
You also won't be able to use shops most of the time - maybe if you find some sort of black market/human dealer who will sell to you at uncomfortable prices.
Perhaps some monsters could gain experience/small bonuses from destroying advanced equipment, especially those that oppose civilization.
As for artifacts - I'm thinking there should be at least one per monstrous talent tree, which should at least increase your mastery multiplier.
Should probably drop from a suitable unique monster, though - also, might not be be equipment, but a consumable item(like the Sandworm heart in ToMe).
STORYLINE/QUESTS/DUNGEONS
Haven't worked on dungeons much.
Maybe you can pick one of several locations, or you start at the bottom of a dungeon and have to work your way up to the surface - most monsters you find along the way will be neutral, so it shouldn't be too much trouble to survive, although, as you get closer to the surface, you might have to face adventurers(mostly human, but you never know).
After that...maybe you could pillage towns, try to become accepted into human society, visit the Lairs of other monsters to challenge them for supremacy.
I'd also like some kind of semirandom quest, perhaps named heroes/heroines who decided to hunt you down and have a chance of carrying an artifact.
NOTES/OTHER INFO
Unlocks: Won't have those, I don't particularly like them and I'd rather give everyone access to everything from the start.
Achievements: *Might* put some in, as long as they're not too complex to add.
Game Balance: This being a singleplayer game, it's not particularly important. I'm perfectly ok with some traits/kins/talents being more powerful than others(as you may have noticed looking at the perks and flaws).
And...that's it, for now. Suggestions would be appreciated!
last edit: cleaned up talents, skills