It’s First Past The Post that forces the two party system. Get rid of that, STV is what will bring lasting change and reduce the the tendency for the two major parties to be exaggerated opposites of each other.
For the presidential elections approval voting would be more appropriate, for local representatives proportional representation is the important part for which you don't really need STV (although I guess voting for political parties with proportional representation is somewhat similar to STV with a predetermined set of preferences).
