Is a "West Marches" style game viable in the PbP format?
I am currently running on RPoL a few campaigns that, while settings and worlds do change, retain some of that concepts of the West Marches, but not all, and they seem to work and keep themselves fresh during the months and years that are required in RPoL to run campaigns, with new players joining when old ones look for different games as normal.
What I made different is that the game is not 100% sandbox, but every time you run an "adventure" that you can choose among many options, or you can ask for a specific kind of adventure (of course you can always go off on a tangent, but in general you have a thread that you can follow) and then there is a "fixed time" for every adventure, before going back to base. This makes sure that all new adventures start periodically at the same time, so the characters can always mix up after each adventure.