My games have always been single player browser games but this jam has made me think about how I might make them multi-player. I have written some design ideas that can be read here (github). Any comments?

I think for a fast paced multiplayer game websockets are probably a better solution, http fetches would be slower and websockets allow push/pull communication. If it was a turn based sort of game then what you propose may be enough. Fetch is still fine for one-off steps (e.g. get player list on join, register zones, or occasional refresh). You should probably consider using a db for data (even sqlite at first) rather than a file and flock() too as it may not deal with concurrency as well.