Creating new ancestries and archetypes is simply a matter of giving them a name, a purpose in the world, and a way to spend Akasha. There’s really nothing more to it than that. However, there are a couple of things to consider.
First, make sure that the purpose and Akasha use of the new ancestry or archetype is not already covered by an existing one. You don’t need another archetype that fights well and uses Akasha to smite. Likewise, you don’t need another ancestry that changes their shape. Make everything unique and special.
Second, make sure the new ancestry or archetype isn’t too powerful or too weak when compared with the existing ones. If it’s too weak, no one will want to play it, and if it’s too powerful, no one will want to play anything else. Keep it all balanced. Here are two examples you can follow.
Nuanced Bad Guys
Exterminis: Reign of Destruction is designed so that players don’t have to face moral quandaries while playing the game. Demons and Undead are completely and irredeemably evil, and Warpchanged animals are in so much pain that it is a mercy to dispatch them. However, this doesn’t have to be the case. If you want a deeper and more nuanced world (as well as an easy way to introduce new character options), you can change things to allow for this. In your world, perhaps most demons are evil, but a select few have learned to rebel against their masters, and are now fighting for freedom and peace. Likewise, perhaps many undead still retain a spark of their soul and make their own decisions. Finally, maybe the Warp doesn’t always cause intense pain to the animals it touches, and the change is sometimes peaceful and beneficial.
New Ancestry
Goblins
Small, Crafty Troublemakers
Goblins are small, green-skinned humanoids who love to get in trouble and cause chaos. They are experts at hiding and all shadowy arts. Goblins are very weak compared to the other ancestries, and so they never travel alone. Where one goblin is found, others are surely nearby.
Leader of the Pack: For one Akasha, a goblin may summon another 1d4 goblins from the shadows. These goblins magically appear regardless of the environment and are always extremely loyal to the goblin that called to them for aid.
New Archetype
Druid
Protectors of Nature and Balance
Druids are eccentric loners who dwell in the wilderness, far away from civilization. They spend most of their time communing with nature, conversing and befriending animals, and repairing the damage caused to the natural world by the Warp.
Detect Warp: For one Akasha, Druids can detect the presence of the Warp in the surrounding area. This requires a Willpower challenge. On a strong success, they detect all Warp in a Far Circle. On a weak success, they detect all Warp in a Near Circle, and on a failure, they don’t detect any Warp. This is the only way anyone can detect the presence of the Warp without direct contact.
Remove Corruption: For one Akasha per rank of the animal, Druids can remove the effects of the Warp on Warpchanged animals, reverting them back to their natural state, healing their pain, and restoring their sanity. This requires a Willpower challenge. On a strong or weak success, the animal is healed, but on a failure, it is not.