N. lowii x ephippiata is such a cool cross, not only in reality, but also in theory. It's not very often that we get to see what two species that are so closely related will produce when they are crossed. I think as plants like yours grow older they will really give us insight into what it is that defines a particular Nepenthes species.
Some other really interesting crosses in the same vein would be something like -
N. hamata x RHH
N. attenboroughii x palawanensis
N. edwardsiana x macrophylla
Really impressive collection though, excellent work with all the genre.