I wanted to create this, but never really had the time.
A plugin that any other custom map and just copy into their map, to give some extra fun and played time. The plugin should allow the player to get a small companion critter to follow him around, without it affecting gameplay. Then in a sims/tamachoti style the player should be able to interact with his pet in various ways, trying to keep the pet happy. Unlockable pets as the player spends more time with a pet active. Cross map stats, meaning the shared bank file would mean no matter what custom game you play (that supports it) you can bring your pet into.
I already created a large amount of the code, if anyone want to take over, or create this.