The Flesh Descends
Intelligent Humanoid: Zern (3.5 Monster Manual IV)
Undead: Leftover Special (Eclipse Phase X-Risks)
Ancient: Wastewalker (Eclipse Phase X-Risks)
Big & Angry: Slouch (Eclipse Phase X-Risks)
Lizard-thing: Wedon (The Apocalyptic Post, Issue 2)
Aerial: Aeronautilus (The Apocalyptic Post, Issue 3)
Aquatic: Mud Puppie (The Apocalyptic Post, Issue 1)
Extradimensional: Spirrax (3.5 Monster Manual V)
Unpleasant & Underground: Blister Beast (Eclipse Phase X-Risks)
So, having taken monsters mostly from The Apocalyptic Post (a Gamma World zine) and Eclipse Phase, the obvious choice would be a sci-fi setting.
So let's do the opposite.
Transhumanity has been gone for a very long time. A thousand thousand years ago, they opened up a gate to a higher dimension, and went through it.
That gate killed the world. The transhumans who stayed became Wastewalkers - masked terrors from a prior age. Less than a hundred still survive, leading nations with their advanced, augmented minds. The Zern wield the arcanotech of the Wastewalkers, but have forgotten what they were for - the Wastewalkers' bioconstructs have been torn apart and put back together into the horrific Leftover Specials.
The Spirrax came down from the gate the transhumans opened - as the Aeronautiluses scout for transhuman technology, the Spirrax descend and consume it before grinding spirals into the earth as summoning sigils for the Hydra - manifestations of the unholy will of the Spirrax. Magic comes from the Sigilblooded, who have spent enough time around the Spirrax for their will to be enhanced in the same way.
Algos, the largest nation of this world, is trapped between long mountain ranges infested with burrowing Blister Beasts. The swamp to the south is covered in the quick-breeding amphibian Mud Puppies, preying on the large Wedon - their emotion-manipulating pheromones make them unassailable to the Zern. Only the Wastewalkers, with their synthetic minds, can hunt them.
The capital city of Algos is heavily fortified, with thick walls culminating in a high iron dome. The borders of the city are patrolled by Slouches - large automatons built by pre-apocalypse Wastewalkers.
1. A cache of transhuman technology has been found in the swamps below Algos - can you reach it before the Spirrax descend and consume it?
2. An earthquake has collapsed a trade route - anyone moving across it will fall into a nest of Blister Beasts.
3. The ceiling above the capital has collapsed. Now, the Aeronautiluses can see into the city. They can see the technology stored within. A Spirrax will soon descend and tear the city apart.
4. You have been found guilty - your punishment is to kill a Wedon. But how can you kill it without getting near it?
5. Leftover Specials are starting to decay - with their minds slowing and muscles stopping, the labor force will soon collapse. Some Zern are planning to create more. By kidnapping the living.
6. A Wastewalker has assembled an army, and is moving towards the city. The Slouches will obey his commands just as they obey the leaders of Algos.