Prequel probably has the highest chance of succeeding and doing well. Reboot would probably be the easiest, but would be a little boring. Sequel is a no no, that's sketchy ground to be going into.
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Exactly what I would of said. I don't see it being a sequel, but one of the other sources suggested it can work (Of course I don't think Watchmen should advanced forward. Some series are best left as they are).