I’m not sure a third party will emerge in the near (or reasonably far) future. One reason: The parties are pretty good at adapting to ideological changes on the ground. (This, partly, explains how the Democrats are now the liberal party after being the conservative party for much of history.) You can see the changes that occurred in the platforms at the DNC and RNC, for example, or how Republicans are becoming more Trumpian re: free trade, etc.