How about the old Kirk as a villain. From Shatner's book The Return. Kirk is reanimated by the borg and Romulans but programmed to bring down the Federation. Great way to bring Shatner back in and end his character properly maybe by ending him at the end in a fitting battle to finally save the federation he was sent out to destroy or bring down.
I'm sorry but that is WAY to fanfanky for me