Some things I'd like to develop...

Classes

• Fighting Men - Demon Hunters/Samurai (catch-all warriors).
• Psionnes - The "magic" users Those who specialize in mastering psionics.
• Hybrids (Elves in Swords & Wizardry) - partially mutated humans who will inevitably "turn" but have a wide range of abilities that they can use to help the humans in the meantime.

Settings and Societies

• A spirit city populated entirely by ghosts.
• Lairs for the spheres and spawn points from which their monstrosities emerge.
• City of the damned - a city that has been completely "turned" (e.g., mutated/corrupted) by a sphere.
• Pockets of unreality that have emerged on the planet.
• Roaming warlord tribes.

Monsters

• Re-animated dead (including prehistoric creatures).
• Mutants (all types).
• Unterrestrial (totally new/alien to Ketoru).
• Spirits/Demons (in the Japanese sense).
• Kaiju (Referred to as "gods").

Other

• Tips of forming hunting parties, what type of roles need to be filled.
• Gazetter-style updates on sightings of the Spheres, monsters, treasures, astronomical observatios relating to the Spheres' appearances and the coming supernova, etc. for adventure hooks.
• Possible history of Ketoru and where some of the re-animated originate from.

Some quick campaign notes on influences

Princess Mononoke - Ecological disasters create monsters. Monsters fed from polluted chi energy.

Ghostbusters - Demon hunters/exterminators, problem solvers.

Spirited Away - some spirits/monsters seek nonviolent resolution and may even end up helping in fight against the spheres. Memories of the long past chain spirits to Ketoru, preventing them from finding peace and getting them mixed up in situations with the living.

2001: A Space Odyssey - Spheres are like the monolith, having an effect on the human population (re-animate dead, mutatation, causing changes to physical reality, blending physical and spiritual world, opening dimensional gateways)

A Princess of Mars - Planet is post-apocalyptic society in the far future that's fallen into barbarism. Barbarian warlords rule the day.

Smallville - Humans are undergoing the effects of the Sphere's mutational and warping powers (much like kryptonite affected the citizens of Smallville). Cases of "infection" crop up that cause problems for our heroes.

Psionics - The base type of magic. No purely-fantasy based magic exists. Rip out the spells in R&R, replace with psionics (in-game PCs would refer to it as "magic" just the same).

Re-animation of the dead - Zombies, wights, skeletons that roam and attack (or sometimes don't attack) without a base-alignment. Sometimes they even attack each other. They're more like terrain to be encountered--not an fighting force that can be organized.

Ancient technology - Level of technology is mainly barbarian/medieval but occassionally something form the long past is recovered--and ancient treasure. This could be something from our modern world or even more advanced that appears to be completely "magical" to the people of Ketoru.