I don't see them ever becoming a worthy third party, but I would say they need to get most of the mountain west, the Midwest, and much of the south (like FL, VA, TX) to win, which I don't see happening.
For the Libertarians to become a viable 3rd party, they need to be more like Gary Johnson than the Paul family.
Indeed, the Paul's, while appealing, are more like libertarian republicans than actual libertarians. Gary Johnson doesn't appeal easily to voters but can actually get many progressives on his side if the democrat isn't motivating. A Reason poll showed that 60% of liberals would support a fiscally conservative socially liberal candidate, while only 43% of conservatives would do so.