Useful links - 3lijah/BrowserQuest GitHub Wiki

Design Patterns of Successful Role-Playing Games - 272 pages. But seems like worth a skim.
Open Game Systems - Potential sources of inspiration. Stick with the Creative Commons "(Attribution)" and "(Attribution/Share-Alike)" ones. Avoid anything using any form of "Non-Commercial".
OpenGameArt.org - Possibly useful source for further artwork.
BrowserQuest Class Diagram - Shows inheritance between all classes.