The fabulous thing about ARG’s is that you can let your imagination run free. It can span galaxies (just make sure that all the actions occur on Earth) or be as simple as solving a murder.
I like to use SpiderScribe™ software to keep track of all sorts of things from names of NPC (Non-Player Characters) to the broader picture of the ARG.
Try it! I promise you’ll never be disappointed.
It doesn’t have to be that software, it has to be flexible and robust enough to handle the job (like Adobe xD™, Samepage™. Flock™. etc.), it can be done with Google™ Docs/Drive or something you’re most comfortable with.