A lot of good suggestions here. I would really like for them to build on existing occult packs more. Give more spells to spellcasters and more occult interactions as well as more focus on how the other occults interact with each other. I feel this gets glossed over all the time.
Make aliens better! They could do some really cool interactions with Sims, especially those who are secret agents, police, and scientists. Show common interests or give special items or info to astronauts. Have more interactions based on confusion about human things or mocking intelligence. Also, better CAS options for them to not look so human-like, like horns, skin texture/details, tentacles, and different, longer nails. Better teeth options too.
For vampires, add a lot type for plasma donation where Sims can sell their plasma and vampires can buy it. Also, it wouldn't hurt to make the vampires more scary and lethal (getting Sims the bite, sick, or death). They are very watered down, which doesn't make them as fun to play.
I play a lot with the occults (including plant Sims) and they are seriously lacking in all areas.