We have several specific third-party markets, but I would like to see one that asks whether there will be a viable third-party candidate on the November ballot. There are several ways of asking this.
I think it would be interesting to have a market for something along these lines: how many presidential candidates will appear on the ballot in at least 45 states for the general election? Choices: 2, 3, 4+.
Or possibly a different way to get toward the same kind of question: How many candidates will receive at least 1.5 million popular votes in the general election?