Looks as though there may be a better road on the way to Puerto Vallarta (there are already 2 or 3 that do not go as far as Tepic) but we haven't yet heard information about a good road past PV. This is a difficult area because of the mountains that rise around the southern area of that city. The road to San Sebastien is accessed somewhere near the airport, to go east where it leads one to the Guadaljara region via a connection to Hwy 80. That stretch of road is very steep in places and prone to slide activity in wet weather, so not really a logical route to replace Hwy 200, plus, you'd still have to deal with some of PV's traffic.
I can see what you're looking for Robathelake, more of a straight shoot off Hwy 15 that arrives on the Costa Alegra, no cities to go near.
With the grand expectation of the new airport being completed south of Tomatlan, and nearly all the fine beaches now "spoken for" to the exclusion of folks driving on Mexico 200, the area is "slated" to become a big, Fly- In destination. Of course, Melaque is closer to La Manzanillo airport, but if flying in to the north, what hope would there be for tourists to leave their hotels. We saw first hand the difficulty visitors to Blue Bay resort always had to see the towns to the south, like Barra de Navidad.