If Thrall dies, it then is a carbon copy (almost literally) of his own parents story thrust onto his new child. Of the horde being in a dark place and the parents trying to save their people from them selves only to be murdered and leaving the child with out a father/mother
If Aggra dies and thrall lives. Its too similar to Varian and his sons story of their wife/mother dieing, and how he over comes this anguish to lead his people.
Thrall should live.
If anyone should die to thrust the horde into action it should be saurfang. He should die a heroes death. He should come down from northrend hearing how Garrosh has lead his people down the old dark path. Challenge him for leadership and die after putting up a heroic effort. Garrosh then showing his new found power/corruption shocks members of the horde and with the news of one of the great heroes of the horde being killed splits opinion among the horde members specifically the Orcs.