He has to die in EP III. Yoda said to Luke "the last of the Jedi will you be" which means Mace is dead. He'll die on screen, just like Padme. These characters are too important not to have any closure on why they are not in the OT. I'd like to see Anakin or Palpatine use some dark side to take Mace down. I really hope Mace dies protecting either Yoda or the twins or maybe even both.