So after the introduction of Vader's castle on Mustafar in Rogue One I was left pondering on this topic. The castle was initially part of the episode seven concept art, which was rejected and then recycled for Rogue One. Initially it would have served as a forward command base for the resistance - which I'm personally glad that they did reject. In the RO visual dictionary it claims that some parts of the castle are not 'yet' seen, which presents the possibility that we will see this later on. In terms of contribution to the plot I think it will definitely help to develop Kylo Rens obsession with Vader. It is also built over a sith temple or more an important sith place so that influence on Kylo Ren's training could help him develop. Also Snoke says to general Hux at the end of TFA "Leave the base at once and come to me with Kylo Ren, it is time, to complete his training." So will Snoke be living in Vader's castle? If so why? One explanation is that Snoke is Vaneé (Vader's servant in RO). This would make sense as he would know the castle well and all of the sith artefacts there (supposing there are any) and so could teach Kylo from them. Plus it is clear that Vaneé is not a sith neither is Snoke. It is implied but never explicitly stated that he is force sensitive, being referred to as 'wise' but not powerful. Infact Han says that Snoke is using Kylo for his power. This may show that he isn't force sensitive at all. Finally Vaneé is somewhat compatible to Yupe Tashu from 'Aftermath'. Yupe worships and pursues the Dark side and the sith but has no actual force abilities, being one of the emporers advisers.