Sandman is definitely going to try and stab Oda in the near future because he thought he was his friend or some crazy stalker shit.
Option A: We find out Sandman has an Oda bodypillow or sexdoll or collects his discarded trash (we already know he keeps track of Oda's diet and sleeping habits and knows where he lives).
Option B: Sandman sneaks into Oda's apartment one night to try and steal a lock of his hair. Insanity ensues.
Option C: He plants CP in Oda's apartment and calls the cops on him in revenge for never answering his letters.
Option D: Stabbing insanity.
Option W (Worst Case Scenario): He finds out something questionable about Oda and uses it to blackmail him into being his "friend" but when it eventually goes too far, Oda decides to take the risk and go public about it, sending the fandom into a chaos it might never recover from and Sandman commits seppuku.
Dude's even visiting Oda's hometown and talking to everyone he ever knew. That's messed up.