Picard or Shinzon, with the audience left wondering.
That is, have Stewart portray both characters (at least after Shinzon's weird biological processes reach their peak - Tom Hardy until then is fine with me), have the big fight between the two in the end, and have a person looking like Stewart sacrifice his life in order to stop the evil thing (loss of E-E, or perhaps loss of Earth) from happening, while another person looking like Stewart survives, walks away from the carnage, and sits in Picard's chair. Did Picard triumph, or did Shinzon realize how wrong he had been, have a change of heart, and finally assume the role he was destined for - that of replacing Picard?
Everybody expected Baird to pull a Gladiator
with this story; that's why he was asked to do the gig the first place. A happy ending involving the death of the hero would be right up his alley, and what was expected, but with enough of a surprise twist to satisfy the audience.