While adding pets has been talked about without much success due to multiple issues staff seems to have, a simple upgrade to properties would also make sense.
These upgrades would be available to all properties and possibly able to have more pets as you get bigger properties.
Trailer - 1 Cat or Dog (50 happiness per pet) ($250 upkeep a day per one of these animals)
Apartment - Cat and/or Dog plus add option for small tank animal (fish, small reptiles, rodents, $250 upkeep per day) 2 pets available
Semi-Detached House - Add option for medium tank and/or small bird cage (100 happiness, $400 upkeep per day) 3 pets available
Detached House - Add option for large tank and/or medium bird cage (125 happiness, $500 upkeep per day) 3 pets available
Beach House - Add option for salt water tank and/or large bird cage (150 happiness, $750 upkeep per day) 4 pets available
Chalet - Add option for small pond and/or extra large bird cage (200 happiness, $1000 upkeep per day) 4 pets available
Villa - Add option for medium pond (250 happiness, $1500 upkeep per day) 5 pets available
Penthouse - Removes outdoor options (ponds) 6 pets available
Mansion - Add option for large pond and/or aviary (300 happiness, $2500 upkeep per day) 6 pets available
Ranch - Add option for extra large pond and/or small horse stables (350 happiness, $5000 upkeep per day) 6 pets available
Palace - Add option for medium horse stables. (400 happiness, $7500 upkeep per day) At this point the previous animal types would be able to be upgraded. Purebred dogs and cats, exotic small tank animals (100 happiness per animal, $500 upkeep per day), exotic medium tank animals, exotic small birds (200 happiness, $800 upkeep), exotic large tank animals, exotic medium birds (250 happiness, $1000 upkeep), exotic salt water fish, exotic large birds (300 happiness, $1500 upkeep), exotic stock for ponds (doubles happiness and upkeep for each pond size), thoroughbred horses (doubles happiness and upkeep for each stable size) 8 pets available
Castle - Add option for large horse stables (500 happiness, $10000 upkeep per day) 8 pets available
Private Island - Add option for extra large horse stables (750 happiness, $15000 upkeep per day) At this point a new category of pets would also be unlocked. Illegal Animals with the crown jewel being a lion, tiger, or bear (oh, my) cage (Up to 1000 happiness, $50000 upkeep per day) 10 pets available
The price for these upgrades? Points. As you would be basically buying a license for each pet you own. The exception would be illegal Animals, which would open the door to a new crime (well, a call back really). Trafficking. With illegal Animals being unique outcomes only available with the PI upgrade to house them.
Edit: Upkeep for pets could also be kept separate from normal upkeep and could possibly start having negative effects from non-payment a lot faster. Once the cost gets high enough you could lose your pets and have to pay more points to get them back.
Edit: Having your pets taken away due to neglect (non-payment of upkeep) could also incur a massive respect and happiness penalty. No one likes animal abusers, especially not themselves when their animals get taken away.
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