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.
All they have to do to bring back Kirk Prime is this:
Basically some Trek character clones Kirk in body only - not mind, not living. Maybe they were warned with knowledge of the future, or from the Nexus. They do a simultaneous beam-out and switch of dying Kirk under the fallen bridge with the clone Kirk. Dying Kirk is saved with advanced medical help. Clone Kirk is still there for Picard to bury under a pile of stones. That's why dying Kirk said, "Oh my." He became aware of the beam-out attempt, but Picard was unaware. Kirk lives on in Picard's time, but we know not where. Or maybe he was recalled to the Nexus since he was there once and could then choose to exit into any time he wished. Maybe he joined Spock for a while on Romulus. Maybe he jumps in and out of the Nexus every week for a new adventure in a different time period - like Time Tunnel for Star Trek. Who knows.
Hey, that is very good. We could also have another Guardian of Forever he could go to any time period in the past he wanted.