The opportunity exists for a Watchmen Sequel.  The actors and actress are already contractually bound and rights are already owned.  Read more at /film for the details.  So it could be done.

Should it be done?

Watchmen as a work has such a completeness to slap a sequel onto it would do the entire project a injustice.

If Alan Moore were to do a sequel I would love to see something like this.  Use the same setting, make it a generation latter and update the underlining message to match some of the greatest global threats that we face.

Do you think a sequel should be done?  What would be our current doomsday clock?