I think that the risk pool would be large and the members would be local (thus giving more rights to local communities rather than the federal government).
I think it could then encourage cities to be more healthy cities (less pollution, more activities for locals, encourage walking/biking rather than driving) in order to drive down health care costs for their citizens. Being a healthier city would then encourage people to move to there thus increasing the risk pool.
Citizens wouldn't be required to purchase through them but the plans could still be managed by private companies like Aetna, Cigna, etc. so that the plans could be used when local citizens travel to other parts of the country.