OK, so could you please tell me which of Shakespeare's descendents would own the copyright to the Tempest? Lord knows I wouldn't want to deprive someone of their due. I'm sure you'd like to assist in tracking down the descendents of the Brothers Grimm as well. Lots of their work is being used gratis these days.
If none of them have this work registered or have legal representation, why bother? There may be published versions of this work, however, that you are not free to copy and sell to others. Is that your aim?