If you analyze the central message of "Wandering Willie's Tale" you can definitely extrapolate the sub-theme of "nostalgia" from within the constantly present theme of the old versus the new world, and how values change from generation to generation. Yet, the multilayered message is that money is the source of all evil.
The narrative of the story is told by Wandering Willie, who is the town's whimsical storyteller that has seen and known everything and everyone. He is the link between the world of Sir Robert Redgauntlet and the world of Sir John. The later's was a world of religious persecution, chivalrous loyalty (Steenie was the loyal bagpiper and tenant of Sir Robert), and endless friendships. He is also the link between the natural and the supernatural world (through his story), as he tells of how Steenie purportedly had an encounter with the supernatural in order to fix his money problem.
The theme of the change of worlds and values is evident when the religious persecution of Presbyterians stopped, and Sir Robert's political party was no longer in power. The man lost most most of his fortune, which led him to end the world of chivalry and enter the world of feudalism and rank-survival. He turns against Steenie, albeit begrudgingly, and cannot even forgive the man for falling two months behind in his rent. It is arguably that the fact that he does not fit in this new, cutthroat world is what ultimately renders him weak, sick, and then he ends up dying. His son is no different.
(Sir John) If I understand your trick, sir, you want to take advantage of some malicious reports concerning things in this family, and particularly respecting my father’s sudden death, thereby to cheat me out of the money, and perhaps take away my character by insinuating that I have received the rent I am demanding.
Therefore, what is the causative factor that seems to be ruining everything? It is money (or the lackthere of), even more so than power).
When Steenie has to figure out a way to prove his case to Sir John, his solution comes in the form of....evil! A mysterious meeting with mysterious characters, including Sir Robert's ghost, ultimately gives him the answer that he needs to resolve his situation.
When he gets back with his much needed evidence, Sir John is not neccesarily too happy to hear that his father is not in heaven, but is shock to find out that the money that was hiding was in the precise location that Steenie was told by the enigmatic ghosts that it was. Therefore, money is a catalyst that makes things go badly if there is not enough of it.