Ahoy there!
I am now pleased to announce that my Sea of Thieves fansite boasts what I believe may be the most comprehensive guide to Gold Hoarder riddles anywhere online. I have mapped and photographed over 260 landmarks in the "old world" that feature in GH riddle clues.
I do not claim to have discovered every riddle landmark. So, if you know of any I have missed (in the old world), please let me know in a response below. Riddle clues in the Devil's Roar will come as I discover and manage to document them (between volcanic eruptions).
The "Riddle Finder" page is at http://www.surveyorpete.fun/Tools/Riddles