Interesting thought. I'm not sure that any ever would though. Parasitic plants are parasitic because they can't photosynthesize or gather their own nutrients. Nepenthes can do both of those on their own already so I'm not sure that any nepenthes that developed these mutations would have enough of competitive edge to develop into a parasite in the next hundred million years. If they did they'd likely change so much we wouldn't recognize them as being nepenthes anyway.
I like to think of it as more of a mutualismSure, they can be parasites; these damn plants have been living off of me for years . . .
There are, in fact, epiphytic parasites--mistletoe comes to mind, and dodder. Nepenthes are interesting, too, because the reason they're carnivorous is because of how few nutrients they have. So it could definitely be advantageous for them to not have to invest so much energy in carnivory. However, they would be unrecognizable as Nepenthes in that they would lack pitchers.
It's worth noting, by the way, that all vines are parasites (they compete with their host for light and increase the host's disease load) and most Nepenthes vine. So most Nepenthes are parasites, though not in the way you mean.
Epiphytes aren't parasites because they don't negatively affect their host. They're commensals. I'd say vines are parasites as soon as they start to negatively affect their hosts (which they often do), because they benefit at the host's expense. But at some point it becomes semantic.Technically vines are not parasites. The definition of a parasite does not fit in with that of epiphytes. Also, I was referring only to the family of orchids on epiphytic parasites, not all plants.
Adding on to Schmiggles comment, there wouldn't really be a reason for a parasitic plant to be carnivorous as it wouldn't need the additional nutrients. I didn't really think of that until just now
Epiphytes aren't parasites because they don't negatively affect their host. They're commensals. I'd say vines are parasites as soon as they start to negatively affect their hosts (which they often do), because they benefit at the host's expense. But at some point it becomes semantic.
The thing with carnivorous plants is that it takes a huge amount of energy to be carnivorous, which is why they fair poorly against pretty much anything else if it can survive in an area. Otherwise I'm sure they would be more widespread. So if a carnivore could be a parasite and not put as much energy into producing traps, it would have a huge competitive advantage. I suppose the only reason that hasn't happened is because any intermediate state would be hard to come by, given how specialized epiphytic Nepenthes are to carnivory or other forms of nutrient acquisition through the pitcher.