I was just thinking something similar. I would put more emphasis on the volunteering, as petition based stuff doesn't really improve things. Turn it into a location based community task site.

People and charities can pitch to projects that require volunteers in their area. Each project can be discussed with threads. Could be as simple as "We're landscaping the local park at the weekend, turn up bring the kids and help" one day stuff, to "shelter needs volunteers for ongoing stuff". The project could be discussed and planned in their own threads, and users gain karma for their volunteer work.