Being totally new here, I'm sure this has probably been mentioned long ago before, but I didn't see it after a few forum pages so I thought I'd mention it anyway.
A couple of things I would like to see would be the ability for Group cats to interact with each other, instead of just Owner cats having that ability. Also, the ability of Group members to be able to raise a Group cat's Happiness by interacting with it.
Maybe having it as an option within each cat's menu, so the Owner can decide if they want to allow this or not. It's already possible to set Food so only a Group cat can use it. An expansion of this to the cats themselves would really be nice.
Group Cats Interactions/Happiness Levels
