Animal Rescue Pack...

If you have Cats & Dogs then your game has all these sick strays wandering around. You can befriend them and give them a vet treat, but they will just be sick again after the next loading screen.

The Animal Rescue pack could let us befriend, collect, treat/rehabilitate and adopt strays to NPC households. This pack could also be used to bridge between Cats & Dogs, Cottage Living, Horse Ranch and Werewolves (you know you want to take your werewolf to the vet). 

If you have Cats & Dogs you can use the Vet career to treat animals. If you don't have Cats & Dogs but do have one of the other animal related packs a service npc vet could be added. It would work like maids, gardeners and ranch hands. You call them on the phone, they treat the animals and leave. Being treated by a vet could extend the lives of all types of animals in the sims. 

Now with Businesses and Hobbies a player could play a large rescue entity like the Humane Society or a small residential rescue. Each with their own lot types.