The Inheritor, or the Extremely Omnivorous Blue Mage

The Inheritor is based heavily on Courtney's Blue Mage except with the vore stuff cranked up to maximum.
In essence - eat monsters to steal their abilities.

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To die beneath the harsh eldritch light of the Black Sun is to be born anew. Soul scourged, memories scattered, barely in control of this new body of black-veined mud.
The life of a newly formed demon is one of sin-eating, slow-gained sentience, and hard-won metamorphosis. A new body with an old soul, emerging into the darkness of the end of days.

But this is not you.

You are an aberration amongst even your own aberrant kind. You crawled from the viscous mud of the river Lethe fully-formed, already settling into shape. Stranger still, a weird grasp of your own twisted flesh allows you to alter your body further. A malleable self-image that can be restructured to assimilate the organs and energies of foes you confront and devour.
You are no mere demon. You are something more.

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The Inheritor

Core: 1d6 Hit Die. Minimum 3 HP at first level. Saves and Exp Track of the Magic-User.

Monstrovore: The Inheritor is a strange type of demon that can devour monsters to gain their powers.
A sort of monstrous and extremely omnivorous Blue Mage.

Mutable: In order to absorb a power or ability, the Inheritor must have been affected by that power or ability. Then they must eat whatever part of the creature gave them that ability, like a ghoul’s claws for paralysis or spider’s fangs for their poison bite.
That is, to steal a powerful attack you must survive that powerful attack. Then eat them.
In the case of passive abilities, like a Spider’s climbing or a Fire Beetle’s glow, you can steal the ability as long as you’ve witnessed it in action.
During the absorption process the Inheritor collapses into a pile of randomly mutating flesh and mud and strange organs. This process takes 10 minutes.
You can take multiple abilities from the same creature if you wish, but each individual ability is a separate Mutation.

Inheritance: Each ability the Inheritor has stolen is called a Mutation. Each Mutation grants the Inheritor a minor passive ability based on the Mutation. Taking a Giant Spider’s climbing ability might give a passive +1 to Climbing, taking a Gelatinous Cube’s paralyzing touch might grant soporific saliva.
The Inheritor’s Mutation Capacity is equal to their level, eg. a level 3 Inheritor has 3 slots for Mutations.
The Inheritor’s maximum MP is double their level. They regain all lost MP after 6 hours sleep.

Full Power: Spending a Meat Point (MP) allows the Inheritor to use a Mutation at full power for a round, using it as the original creature used it. This causes their flesh to bubble and unfold into a monstrous parody of the original creature as they unleash the stored power.
Other than available MP, there is no limit to the number of powers that can be activated simultaneously.

Doppelganger: The Inheritor can entirely consume a person to gain their voice and appearance. Each whole disguise  counts as 1 Mutation. It costs 1 MP and 10 minutes to transform, but the transformation is permanent until you transform back.
You also get a vague taste of their memories - ask the DM three questions and he must reply truthfully if they’d have known the answer.

"I'm spending a Meat Point to run as fast as a dog!"

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Core:
The base of this class is a Magic-User with a bumped up Hit Die.

Monstrovore: 

You get it. Eat monsters, gain their abilities.

Mutable:

The idea of a PC going around collecting monster abilities is a little bit scary for me because my game has lots of weirdo monsters with weirdo powers. What if they pick up some completely overpowered ability and break my game?!?

There are three main risk/reward aspects to collecting monster abilities:

1. You have to survive a powerful offensive ability in order to obtain it. If you want that dragon's breath weapon, you have to survive the dragon's breath weapon.
2. You have kill and eat the powerful monster. If you want that dragon's breath weapon you're going to have to actually slay the dragon which is no mean feat.
3. It takes a 10 minute Turn to assimilate monster abilities, so you need to be protected until you've fully absorbed the creature's ability.

This means that if the Inheritor picks up a powerful ability, they've earned it.
It's already getting a little Pokemon with swapping out moves for more powerful moves over time, but I'm more than ok with that!

To be clear - you can steal multiple abilities at once from the same creature. So if you eat a spider you could steal its Venomous Bite and its Spider Climb.
Bear in mind that each distinct ability takes up its own slot.

Inheritance: 

This is obviously the main gimmick. Steal an ability and you get a passive perk which is always on.

As an example, at time of writing the Inheritor in my game is at level 4.
She's filled her 4 Mutation Slots with the following abilities and associated passives:

1. Fire Beetle glow (Glow faintly in darkness)
2. Dwarf darkvision (Low light vision)
3. Giant Spider venomous bite (Paralysing bite)
4. Gelatinous Cube transparency (+1 Stealth due to mild translucency)

Each of these passives has made her a little more mutated. The fire beetle glow comes from softly glowing orbs under the skin, the cube's translucency has made her a little more squat and slimy.

Full Power:
The resource management part of the class. You get two Meat Points per day per level that you use to amplify a stolen ability back to full power!

Spending a Meat Point instantaneously gives you access to the full active version of the ability for the rest of the round.

For this Inheritor, spending a Meat Point will stoke that faint glow to full lantern-strength light, or she can activate full power Gelatinous Cube transparency to go practically invisible.
Of course the big ticket item here is the Giant Spider's poisonous bite - spend a Meat Point and you can bite with a spider's devastating 1d6 damage bite and Save vs Doom poison.

She did well to acquire that one.

A single Meat Point will keep an ability at full power for only a single round, so some abilities are more useful as a passive than an active ability.
Bumping Fire Beetle glow up to full torchlight lasts for a full round, but one round of lantern-strength light is not so good for exploration over the course of ten minute Turns.

Cleverly this Inheritor has exploited the passives to create a better combined effect, something I hadn't initially expected. 

You may notice that the combination of Fire Beetle soft glow and Dwarf low light vision will combine to result in constant passive darkvision. I thought that was real neat! Good use of player skill and exploiting edge cases, so it's perfect.

Doppelganger:

Straight out of Prototype, eat a person to become a perfect clone of them.
This is ripe for shenanigans, and also means that an Inheritor who is willing to use up a Mutation Slot on a normal person disguise can avoid looking like a weird horror-chimera if they want.

The first use of this ability was pretty fucking wacky - the player ate the corpse of their previous character in order to transform into her.
The ability got swapped out for another power fairly quickly, but it was creepy while it lasted. Especially for the other characters who had buried her.