You might like to base it on a book (or movie) that you’ve seen, or base it from your own personal experiences (for example, I could base it on my Peace Corps experiences). Basically, any idea that you have that intrigues you could be fodder for an ARG.
Be sure there is something to please as large an audience as possible. You could base it on a popular; singer, actor, current event, television personality, etc. You could run the idea by friends (be sure to thank them!) they may have some good ideas that they think should be incorporated.
I find that the more flexibility there is the better. Be sure that it’s flexible enough to admit more players. You could even “catch them up” by posting a where are we now? on the main website (or whatever way you chose communicate with the players). Or you could run an asynchronous ARG (more on that later).
This is not intended to be a complete guide, just some tips.