That is quite a list with many good suggestions. I like to play with occults and also see some room for improvement
Spellcasters
- allow the younger ages to practice magic or perform one or two child proof spells
- add in a magic lot trait so spellcasters can behave like spellcasters outside the magic realm
- update familiars to be interactive
- let brooms fly instead of teleport. Now that we have bikes in the game, brooms can use the same code for routing, just a few meters above the ground.
- add a few new spells or potions every now and then. Perhaps even cross pack spells for example a spell to make sims famous, a spell to change the weather, a potion to cure vampires or werewolves,...
- add in a normal sized cauldron
PlantSims
- make them a proper lifestate with unique abilities
- with skill tree,
- spawning plantsim infants with spores,
- allow different appearances for hair style.
Werewolves
- add in a feminine body frame. Even if the fur is so thick, a possibility for a feminine frame would be nice
- update more necklaces and earrings for werewolves
- allow hats for werewolves
Vampires
- allow a toggle for vampire break ins. It is sad that vampires in their dedicated pack were nerfed to suit the realism fans. A toggle wow allow both occult players and hard core realism fans to change it to suit their needs
Mermaids
- allow all ages to have a fish tail. A mermaid without a fish tail is no mermaid at all, even a child
- add in a skill tree
- add a mermaid bassinet and grave stone
- cas: more options for scaled upper bodies
- fins on arms
Servo
- add in a rain proof option (not waterproof, just rain proof is enough)
- allow more variety in appearance. Not just changing colour but also shape
Aliens
- add in a skill tree
- allow us to live on sixam
- add a link between disguise and alien form like we have for vampires, mermaids and werewolves. Even if it would be just for clothes, hair and makeup, it would be very useful
- a few more alien outfits
- alien 'hair' like tentakels, fins,...
All occults
- don't restrict the cas to occult only. Allow pointy ears, fantasy eyes and pastel colored skintones for all Sims.
- some support for hybrids. I know they are not supposed to happen. The amount of possible hybrids in the sims 2 was insane and I can see why you would want to avoid it. But hybrids can happen by accident. So at least give the basics as in a stable UI for needs and an option to access skill trees by clicking on the sims themselves.
- add in occult holiday traditions for players to have seasons. They can be very broad like use occult power tradition (vampires, spellcasters, mermaids,... any occult power) and a change into occult from tradition (vampire uses dark form, alien uses disguise, werewolves transform) Normal sims can ignore this tradition for being normal, just like toddlers ignore a lot of traditions.