I think you are all forgetting a major fact here. At 13 years old (Assuming he was ten in AotC) there is no way in hell he could physically be able to do anything in Episode III. Other than the interesting clone idea, there is nothing for him. Boba's feeling of loss when his father is killed is manifested in his future, following his father's footsteps and wearing his armour despite its withered state in TESB. He may be involved in some degree, I am not discounting it, but I just don't think it is likely. Vader will kill Mace. The only other person who could is Sidious, Dooku will be killed by Anakin as he "fails" his trial and joins the darkside, just as Luke would have to defeat Vader in RotJ to join the Emperor.