Third parties in America, at least in recent history, suffer from a deficit of legitimacy: This as a result of many third parties being a life support system for either a billionaires ego or an activists' rage; from Ross Perot and his Reform Party hijinks and Mike Bloombergs perennial party hopping to Ralph Nader and Jill Steins' hijacking the Green party.
The single biggest reform with maximum possible affect will be the limitation of money in politics and no other reform will be as efficacious as it could be without such reforms.