I'd love to see the ability to create one's own supernatural type, rather than having to rely on preset "species".
I imagine this feature wouldn't produce "hybrid" variations of two species -- hybrids that include all the powers of both -- rather, it would be more of a "select a single x power from x species, a single y power from y species" and come up with something new.
Another variation of this pack is to add very small powers to the pack, in order to have a pool of combinable "supernatural traits" to choose from?
EX: Cosmetics, such as feathered wings for enhanced movement speed, or possible "flying" that could allow the Sim to teleport to a specific spot/maybe teleport to a different lot?
Powers, such as ones that increase the ease in which a Sim learns a certain skill, successfully perform romantic interactions, etc.
Curses(?) that could be your new race's ultimate weakness.
By combining a supernatural "part" , a "power/trait", and a "curse", you could come up with your own kind of supernatural race to use in your Sim world.
Of course, there shouldn't be anything extremely overpowered in the power/trait pool.
That's up to our modding friends, haha. ;D