user: Let's come up with a rich and complex alien world setting suitable for stories, pseudo-documentaries, or roleplaying games such as DnD. This can be an involved process, so I'd like you to just cover the basics first. We can refine the details further later.

I'd like a planet with a variety of biomes, inhabited by many creatures, much like Earth, but life on this world took its own path, with different plants and animals (and perhaps a different distinction than "plant" vs. "animal" to begin with)

There should be at least one sentient species represented in a wide variety of cultures and societies. They should have complex interactions with each other, and other species on the planet, and resulting from that, their own needs and wants and fears which ought to be relatable in context but may at the same time be strange, unusual, and, well, alien to humans.

But let's start at the beginning: Can you give a few broad, interesting biomes and how they might exist in their "wild" (pre-sentient) forms vs. how a local sentient species might have altered them over the ages?
While doing so, it is important to consider the solar system that planet is a part of, as other objects of the system, especially ones that are very close or very massive, may have a significant effect on the planet and its biomes.

assistant: Some of the unusual biomes for fictional alien lifeforms are:

* Sun's Corona. It is speculated that at some point charged plasma particles may exhibit cell-like and dna-like behavior, potentially giving birth to plasma-based lifeform. Such lifeform was featured in a short story "Out From the Sun" by Arthur C. Clarke, published in 1958.
* High pressure environment, such as "surface" or depths of Jupiter. In this environments common gases would turn into metals. Those were featured in "Call me Joe" story by Paul Andersson, written in 1957. 
* Surface of neutron star. This sort of unusual biome was featured in "Dragon's Egg" by Robert L. Forward, written in 1980. 
* Methane world, where Methane is used instead of water. An example of creature from such world was featured in "Snow maiden" short story, written by Kyr Bulychev, in 1973, featuring an alien woman with body temperature below -70 degrees celsius.
* Charged dust clouds. It is speculated that charged dust clouds scattered in space may, in some situation, develop life-like properties, using electric charges.

Note that those sorts of biomes would be largely incompatible with earth-based carbon lifeforms, and would not be suitable for biomes on a single planet.

For planetary biomes, following biomes and evolutionary alterations are possible:

* Low density atmosphere area. It will create lifeforms with increased lung size, and large surface area, assuming the lungs would evolve. 
* Alternative symmetry. Majority of mammals use bilateral symmetry. A biome where radial symmetry is used will result in a vastly different appearance of lifeforms, for example mimicking that of a starfish. Five legged creatures would be possible.
* Different number of limbs. Most mammals possess 4 limbs. In a situation where evolution develop different number of limbs, worlds with centaurs or winged creatures become possible.
* Increased amount of oxygen. Increased presence of oxygen allows larger insects. At the same time, it produces a much more dangerous fires.
* Sunless or distant world. The sun barely gives any energy, and the planet is covered with crystallized gases and rivers of helium, without an atmosphere. Existing creatures would possess heavy insulation, hunt for heat sources.
* Geothermal world. Frozen water world without sun, where life developed in oceans covered with eternal life,' with their only source of power being geothermal power of the planet. May produce civilization of blind creatures, or civilization without astronomy.
* High gravity world. This sort of environment could produce flattened creatures, flight would be unthinkable, and fall would be fatal.
* Low gravity world. This can produce large, fragile creatures capable of floating.

Many other possibilities are possible.

user: Which scenario would make for the most interesting story?

assistant: All of the scenarios mentioned have the potential for interesting stories, but the one that stands out to me is the geothermal world with a civilization of blind creatures. The concept of a civilization without astronomy, where life is dependent on geothermal power and creatures have evolved to live in complete darkness, presents a unique and fascinating environment for storytelling. It allows for the exploration of themes such as survival, adaptation, and the limitations and possibilities of different forms of intelligence. Additionally, the challenges and conflicts that arise from living in such an extreme and isolated environment could lead to a compelling plot.