Some players may want to have witches, vampires, werewolves or plantsims which do not like being abducted or becoming various other creatures, possibly for storyline purposes or simply for more variety. The problem is, these sims may roll fears of becoming creatures, and these fears have a -5,000 aspiration point value - powerful enough to send the sims into aspiration failure if fulfilled.
To prevent aspiration failure, make sure to fulfill as many wants as possible to bring the aspiration level to top platinum; this way, fulfilling a 'become creature' fear will only bring the sim's aspiration level to high green. The sim will of course roll a want to be cured unless the creature he or she has become is a zombie - without hacks or mods, this life state is permanent. These cure-wants, while pesky and can spam the sim's want slots for a long time especially if the sim knows other sims who have changed lifestates recently, can roll away eventually. It takes longer if the sim stays on his or her home-lot for a long time, but these wants will roll away more quickly if you have the sim visit or buy a community lot.
Without the want-fear sanity mod that can be found at the "More Awesome Than You" website, these sims will fear other sims switching lifestates, even to the one the sim is currently at, and will want others to be cured still unless given Knowledge as a secondary aspiration. The Want/Fear Sanity mod will curb this phenomenon.
Another trick players can use is to have a newly-created sim visit community lots until the witch of desired alignment arrives. To expedite this process, make sure your sim has seven or fewer playful personality points, and at least three logic skill points. This will allow him or her to meditate, so your sim's needs will be static until he or she stops. Once you make contact with the witch, talk with her for awhile and then either go home or to another communty lot, and start meditating again. Build up your relationship with the witch; it is best if your sim becomes a good witch or warlock, because the memory for that is positive regardless of aspiration and has far more benefits than being evil. Once your newly-created sim becomes a witch, he or she is unlikely to roll any wants to be cured, especially if Want/Fear sanity is installed; it's similar to how Rose Greenman is fine with being a plantsim, despite not having the Knowledge aspiration.
If there is negative gossip concerning becoming a witch, your sim may develop a fear of turning, so if you want your sim to be magical, you need to have as little contact with other sims as possible, or use the Batbox Lot Debugger from "More Awesome Than You" to clear all gossip. This way, your sim will be less likely to roll fears of turning, as newly-created sims are blank slates; experiences relating to others changing life-states or being abducted, or gossip relating to such events tends to be what unlocks these wants and fears.
For becoming a Vampire, hang around Downtown lots, and either meditate or make sure you are on a lot with an espresso stand, a grill and a bathroom; your sim's hygene will still go down, but critical needs will be taken care of, and your sim can even earn some money by being the barista. If there is a bar or a DJ booth, those can be used for making money as well, though the latter depends on creativity level. This is important if your sim is unable to meditate and you need to keep him or her fed and awake.
If your sim becomes a vampire, you will need to make sure he or she has a night job or is able to sell paintings at home for a living, or work on crafting stations and sell the craftables at home or at a player-owned business lot. If you want your sim to live on forever, this is the way to go.
For a plantsim, just do a lot of gardening and spraying. If your sim has friends, they will get an invisible negative memory of the transformation if they do not have the Knowledge aspiration; that is hard to avoid, and gossip will spread. If you don't wish for that to happen while still wanting your sim to be a plantsim, just have your sim be a recluse for awhile and use the Ecclectic and Enigmatic Energizer to replenish the social motives, or if you have Freetime installed, read a romance novel, as it will do the trick. As a plantsim, your sim will no longer need to eat, sleep or play, but will still need to talk, bathe and go out in the sun. This can be extremely useful if your sim runs a certain type of business, such as a public pool, and sims with the Family aspiration can fulfill their wants for having ten babies very quickly simply by using the self-interaction of 'spawn plant-baby'. Another useful effect is your sim will be able to talk with plants and sell the valuable produce in a player-run business if Open For Business is installed.
For werewolves, make sure you play with the Pack Leader and perhaps buy a pet house, food dish and bone so the wolf will not destroy the furniture or become hungry or tired so soon. This will take a few days, but once your sim is nibbled, despite havng a bad memory of the experience, if Want/Fear Sanity is installed, he or she will be unlikely to roll a want to be cured. If your sim does have the fear of turning, make sure you fulfill a lot of his/her wants to prevent a visit from the Therapist. Being a werewolf will begin a slow personality change, though there are plenty of mods to either prevent or limit it, and this life-state has advantages such having a relationship boost with dogs, being able to stay up all night, pet the skunk with less risk of being sprayed, and fighting burglars.
With these tactics, you can add more variety if you love having different creatures inhabiting your neighbourhood, yet don't wish to have all playable sims have Knowledge as a primary or secondary aspiraton. Another useful trick is to have your sim use the RenuYuSense-Orb while at green or red aspiration, as this runs the chance of giving your sim the Grilled Cheese aspiration. With most of the wants and fears being related to grilled cheese sandwiches, refrigerators, stoves and cooking skillpoints, you can keep your sim happy despite the creature-related events happening to or around him or her.