What hope is given to people in his tomb saying arthur is the "once king and king will be "?

Because he was the greatest king evver

Le Morte D'Arthur (changed to 'The Death of King Arthur' after the death of author) by Thomas Malory speaks about legends Guinevere, Lancelot, King Arthur, Knights of the round table and Merlin.

By saying Arthur is the "once king and king will be", hope that is given to the public in his tomb is that he will rise again. The ideal country and system he represented will be back again and that he will fulfill his duty to Camelot today and tomorrow.