THERE IS NO XP! TAKE MONSTER DRUGS INSTEAD!

Picture this: you and your adventuring band return to town covered in blood, your own and other's, to stride into a low dark building on the wrong side of town.

The large man-ork bouncer recognizes you and holds open the beaded curtains for you to pass. Within, on cushions and rugs, lit by dull light, muted by smoke, lie creatures of every description. Dashing cutthroats with curved daggers in their belts, taking a drag from a hose attached to a jar in which the corpse of a thing that is all fingers floats suspended in green liquid. A mage in kaleidoscopic robes takes of crumbling chunks of a calcified alien brain to chop it up into a fine powder to snort.

You pass all these through a purple curtain, decorated with black eyes. Within, jars full of hideous things line the shelves: Beholder-stalks, Displacer-beast tails, a troll's liver, an Owlbear gizzard, Mind Flayer brains, and more. A small figure in a cloak with spindly figures peaks over the counter at you as you toss your bag of recently collected coin on the counter.

"Welcome back, boys," he says in his crinkling voice, "What'll it be this time?"

In many ways this is just gold to XP but it is fun and completely does away with the abstraction of experience points. Most creatures are not much below their natural limit at an adventuring age. Magic is inimical to the ordinary person's brain so it requires augmentation. True power lies beyond the natural.

The deal is simple: bring back gold to buy monster drugs that give you permanent changes to your stats and abilities. Everything is on the menu and it has a gold cost. You can also hunt monsters and bring back their parts for plenty of coin or try and use their parts to make the drugs yourself. Most monster drugs are not immediately addictive, but the more you take them, the more possible it is that you become addicted. 

This is a rather expensive sort of addiction, though, and it often comes with the side effect of mutation. Each time you have Downtime, you have to purchase and use the drug but this obligatory use comes with no benefits and with all the possibilities for further mutation. If you cannot afford the drug, you must borrow the money to a local crime lord or shady figure.

Here is a table of them:

1d8 Monster Drugs:

1. Troll Liver Needle (500gp): Shoot up some troll liver to gain 1d6 HP added to your total. Each time after the first there is a 1 in 20, increasing by 1 for each additional time, that you take the side effects. The first side effect is addiction, then you also lose 1 Int permanently and your skin takes on an increasingly stony appearance. If this gets to a 10 in 20 chance, you also partially turn to stone in sunlight, taking Disadvantage on all checks and saves while exposed to it.
2. Owl Bear Beak Snuff (500gp): Snort some of this to gain the edge in contests of arms. Gain a +1 to Attack Rolls. Each time after the first there is a 1 in 20, increasing by 1 for each additional time, that you take the side effects. The first side effect is addiction, then the rage of the owl bear starts to cloud your mind, causing you to lose 1 Wis permanently and you start to sprout feathers.
3. Gnome Finger Hookah (250gp): Take a drag of this hookah to gain a +2 to a skill of your choice (DCC style). Each time after the first there is a 1 in 20, increasing by 1 for each additional time, that you take the side effects. The first side effect is addiction, then gnomish anti socialness starts to set in, causing you to lose 1 Cha permanently and you start to take a fascination with making things and hobbies.
4. Mind Flayer Brain (1,000gp): Snort some of this calcified Mind Flayer brain to expand your consciousness, gaining a Wizard spell slot (levelless). Each time after the first there is a 1 in 20, increasing by 1 for each additional time, that you take the side effects. The first side effect is addiction, then the alien physiology of the mind flayer starts eating away at your body, causing you to lose 1 Con permanently and your skin starts to turn a slimy purple. If this gets to a 10 in 20 chance, tentacles begin sprouting from your body. At 15 in 20 you can only feed on brains.
5. Myconid Shrooms (700gp): Eat some of these shrooms to expand your consciousness, gaining a Cleric spell slot (levelless). Each time after the first there is a 1 in 20, increasing by 1 for each additional time, that you take the side effects. The first side effect is addiction, then the transcendent visions of the shrooms begin to eat away at your logical reasoning, causing you to lose 1 Int permanently. You also start to grow mushrooms from your body.
6. Extract of Displacer Beast (1,000gp): Inject this to gain the ability to shift through space. For each time you have used this drug, you can teleport an additional time once per day. You can only teleport to a place within 30 feet that you can see. Each time after the first there is a 1 in 20, increasing by 1 for each additional time, that you take the side effects. The first side effect is addiction, then you start losing control of your teleportation. There is a 1 in 20 chance, increasing by1 each time you roll side effects that when you teleport it will be to a random location of the GM's choosing.
7. Doppelganger Bloodweed (1,000gp): Regular pipe weed soaked in Doppleganger blood. Smoke this to gain the ability to assume the face of someone so long as you have a sample of their dna. You can use this power once per day per additional time you have taken this drug. Each time after the first there is a 1 in 20, increasing by 1 for each additional time, that you take the side effects. The first side effect is addiction, then the memories of people you have transformed into begin to plague you. There is a 1 in 20 chance, increasing by 1 for each additional time you roll side effects that when you transform, you will believe you are the person you have transformed into. 
8. Dragon Scale Powder (1,000gp): Eat this to gain resistance to particular forms of damage based on the color of the dragon scale (Red for fire, white for cold, blue for electricity, ect). Each time you take this drug, reduce damage taken from this source by 2. Each time after the first there is a 1 in 20, increasing by 1 for each additional time, that you take the side effects. The first side effect is addiction, then wyrmish greed begins to take over. There is a 1 in 20, increasing by 1 for each time you roll side effects that your kleptomania lands you in prison during Downtime.