You mentioned mermaids and aliens (who could take control of a Sim by entering their body), but let’s not forget about PlantSims. I think the system needs a complete overhaul to truly support hybrids, with a specific bonus tied to hybrid status and a customizable skill tree made of distinct and coherent power branches. Each Sim shouldn’t be able to unlock every occult branch — only one branch could be passed down (or not) to the next generation.
There should also be a significant increase in the number of available powers, especially considering cross-pack compatibility and possible interactions.
I also feel like occult gameplay often recycles the same ideas, with some elements that don’t make much sense — for example, ghost Sims being able to improve plants as if they were PlantSims. I would’ve much preferred if ghosts could instead temporarily return to life after a Spectral WooHoo.
Back on topic — I agree with a comment I read recently: unless the underwater environment is properly developed, I’d rather see mermaids put on hold for now. Actually, the same goes for PlantSims — there could even be an aquatic plant variant.