It could certainly change the dynamic to have more than two parties in office. But I can't imagine getting a third party in office would come without generational change. Even when there are third parties on the ballot, there is a perception that it is a wasted vote, so people don't bother. Campaign financing and access to the major privately funded public debates is a big part of the issue. When Perot had both, he demonstrated what was possible. But funding is a serious problem and since debates are privately sponsored, the government can't force candidate inclusion.
Maybe I am too dire on the situation. But forced run off elections and guaranteed ballot inclusion don't really go very far to promote change. Money is the problem. And now with corporations opening their pockets to candidates and parties more than ever, the problem seems to be worse than it ever has been.