Speaking of Streusen, Bege comments on him (that he's way stronger than he appears to be and there's some mystery about him) make it seems there's more story to be told there.
My guess is that he will appear again when we get a flashback on the Rocks Pirates.
My guess is that, among known characters, Kurozumi Higurashi, Kurozumi Semimaru, Shaki, Streusen and Ms. Bakkin were all part of Rocks. Maybe some of them could even be "Silver Axe" and "Ochoku".