The client was a player for a DM who hadn't yet mapped their campaign. So we knew some of the names and rough locations but not how they related to one another or what they should look like. So the client suggested we plot their homebrew locations over the map for the Broken Isles in WoW (because they had the same number of "zones" I think). That actually worked out pretty well. I'd've been just fine drawing up a land mass from scratch but adapting the terrain I know quite well from my time as a Tauren Drood was a lot of fun.