Depends on your definition of viable, I suppose. One could argue that Perot and Teddy Roosevelt were "viable" from the perspective that they made waves, even if they didn't really have a chance (although I suppose the same could said be about Nader if that's the bar for being viable). Maybe a better argument is that Lincoln was an alternate candidate, given that the Republicans were only a couple years old when he ran (although of course they rose from the ashes of the Whigs). I think the rise of the GOP is a good argument for not having the parties baked into law, and for keeping them very private. The down side of the open primary system is that it basically enshrines into law that these two parties can't die. I think in a winner take all system that the mathematics show that two parties will always dominate, but it's not a law of the universe that it has to be these two parties. I would love to see the GOP collapse under the gravity of its own bullshit, but that seems unlikely.