As one of the avenue Star Trek could explore when it comes to bringing Shatner's Kirk back, but one option stands out to me far more than the others.

After originating the character in Star Trek movies. He hasn't played the character since 1994's Star Trek Generations, but Chris Pine and Paul Wesley have each stepped into the role of Kirk since Shatner's last appearance. The 94-year-old actor could technically cross paths onscreen with either of his live-action successors, but appearing in a movie alongside Chris Pine would be the smartest move.

William Shatner Meeting Chris Pine’s Captain Kirk Is The Biggest Possible Star Trek Event

I'd love to see Shatner in JJ Abrams' Star Trek 4

There's very little information about Shatner's Star Trek comeback, but there are numerous projects either confirmed or rumored that could accommodate such an event. If he's going to appear in a TV show, then Star Trek: Strange New Worlds makes the most sense (with time travel, of course), but the Star Trek: Starfleet Academy could also be an option (also with time travel). However, both of these would require narrative gymnastics to make it all work. On the other hand, JJ Abrams' Star Trek 4 movie has the framework already in place.

Just as Nimoy appeared onscreen with Kelvin Spock (Zachary Quinto), Shatner could do the same and meet Chris Pine's version of Captain Kirk.

Leonard Nimoy traveling to the Kelvin Timeline and playing Prime Spock in 2009's Star Trek opened the door for very similar instances. So, just as Nimoy appeared onscreen with Kelvin Spock (Zachary Quinto), Shatner could do the same and meet Chris Pine's version of Captain Kirk. Of course, Nimoy's 2009 role established that Pine's Kirk will still grow to look like Shatner, so the veteran actor could also appear as Kelvin Kirk in the future. Either way, showing up in Star Trek 4 is better than doing so in a TV show, as it would have a broader audience.

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Thor in the MCU, this would have been a landmark cinematic event.

I think the only way to top such an idea would be to have Chris Pine team up with the actor he replaced as Captain Kirk. The Kelvin Timeline movies have acquired a following of fans who aren't necessarily invested in the rest of the Star Trek franchise, but I would be surprised if they weren't aware of Shatner's role during the larger saga's inception. Pairing that portion of the fan base with long-standing Trekkies and Shatner's Star Trek comeback would mean the movie being seen by droves of moviegoers.