One good thing about an original, unpublished game world is that players have a great degree of agency to affect the future of the world – there are no sources or novels to try to stay canonical with. The trade off to that is that much of the world is shrouded in mystery and not every race, class, or background is automatically (or ever) available. With over 100,000 words of background, history, and world notes in my sources, some may be possible that are not listed here; feel free to ask or pitch a specific build that seems off the list.
Most common and uncommon backgrounds are allowed. Rare backgrounds, feats, spells, equipment, etc. requires GM permission. One caveat: if a background (or anything else) mentions an area, region, race, or other that is specifically tied to the Pathfinder world of Golarion, we need to discuss the Khallah Vex way to adapt it.
There are some ancestry and class options that either do not exist at all on Khallah Vex or have not yet been revealed.
These Player Core ancestries do not currently exist in a playable capacity on Khallah Vex: Changeling, Dhampir, Dragonblood, Duskwalker, Gnome, Kholo, Kobold, Lizardfolk, Leshy, Nephalim, Ratfolk, Tengu, Tripkee
Catfolk
Dwarf
Elf, Ghost
Elf, Sky
Goblin
Hobgoblin
Halfling
Human (with Half-elf, Half-orc, Half-hobgoblin)
Orc
The only Core class that currently does not have a Khallah Vex parallel is the Monk. Notes on individual classes are here:
Alchemist
Barbarian
Bard
Champion
Cleric
Druid
Fighter
Investigator
Oracle
Ranger
Rogue
Sorcerer
Swashbuckler
Witch
Wizard
Tamiriyan Common, called Viridian in foreign lands (Essodan, The Usian Coast, Krajavar, Tamiriya)
Daitari (Free Cities of Daitari, trade tongue of the East)
Northern (Halakhvane, Tribal Lands of the western peaks, northern islands)
Suonin (Trade tongue of the islands south of Tamiriya)
Dwarven
Goblinoid (Goblins, Hobgoblins)
Orc
Sulhaii (Ghost Elves)