The Sims 3's Loner trait gives you the Too Many People and the Enjoying Solitude moodlets depending on your situation but your Sims still require social interaction. I feel like a social battery system that depletes as you interact with others be an interesting challenge.

Fun Fact from The Sims Wiki

Loners with the insane trait will never need to socialize with other Sims. This is due to the fact that not only will the social need decrease slower because of the loner trait's effect, but also having the insane trait allows them to use the "Talk to Self" interaction to raise their social need in addition to their fun need.