We Need Fairies

Truthfully, before my brain imploded I had thought that perhaps what is needed is a spin-off that focuses on occults. As said, the current platform is built on the severely lacking foundation built back in 2014. Since then, the game has constantly been trying to fix the foundation while still building upon it, thus an incredibly buggy game that is slowly breaking upon itself the more it builds. Anyway, as for that spin-off idea...

"The Sims: Occults"

It would sort of be like how "The Sims: Medieval" is a spin-off game, but it would not be limited to such gameplay or mechanics. The Sims: Occults would start as a Base Game that has all the current Occults, but not in their "fully" playable forms - they would be limited like how Mermaids and Aliens are now. Regular Human Sims will still be about, but they show up far less - just like how Occults aren't all over the place in the current Sims 4 Base Game. There would need to be tweaks and fixes from the current platform we know from the current Sims4, though - like certain Occults being certain statures and having additional features like tails and wings.
You want a fully playable Occult? That is where the Expansion Packs play in. Each Expansion Pack is dedicated to maybe two Occults. In those packs come the skill tree systems, actual cross Occult interactions, needs that are more unique and specific to their occult, lots that pertain to specified occults, etc. You get to really explore the Occult to its full potential.
Game Packs mean while would allow more situational gameplay where things could get quite complicated... I think one would be a pack that would focus on Occult relations. How do the Occults *really* exist with one another? This is where a lot of planning would have to take place on each Occults' strengths and weaknesses as well as finding a balance of them. Sort of a "rock, paper, scissors" sort of thing where certain Occults are more likely to get along well together and others are less likely, thus the case of possible co-existing among the groups to turf wars. 
Since most of the Occults have similar powers and abilities, but in different forms and fashions, there would need to be the deciding of what Occult is strongest in one category and weakest in another. It would make for interesting socializations. You might have a two Occults who have opposite strengths and weaknesses, but become best friends or more because they make up for each other's good and bad points. You could have an Occult Sim who has a bad relation with a Sim from a different Occult and so have to work hard at mastering the powers and abilities that are the weak points of the other Sim in order to not lose should a real fight start. Or you could be the bad guy and do that to bully the "enemy" Sim.
Other game play possibilities like just how being a certain Occult can affect holding various careers, doing hobbies and gaining other skills could be added. Game play with life stages could come in with age specific game packs, like a toddler GP, a child GP, a teen GP and of course a University GP. 
Stuff Packs would just expand upon build, buy and CAS aesthetics and maybe minor gameplay - so no need to think much on those.

Also note, while my head suffered a headache, it came to me. What probably is holding the developers up big time in regards to fairies are the wings. A lot of the environment in the Sims4 is not friendly to large wings protruding from a human-like back. The wings would be constantly clipping through everything. With Werevolves they were able to find a solution for the problems a tail would bring by giving them all small knubby bob-tails. Fairies, however... Now that is a big problem. You can't just make them have near nonexistent wings.