While he ends up playing a key role in the story, Terminator: Dark Fate should have kept Arnold Schwarzenegger's Carl a secret in marketing. Despite the best efforts of subsequent filmmakers, none of the follow-ups to James Cameron's original two Terminator films have been able to recapture the same lightning in a bottle. The later movies range from disappointing (Terminator 3) to outright bad (Genisys), with the last three entries becoming - ironically - a trilogy of failed trilogy starters.
The most recent entry might be one of the most disappointing, if only for the pedigree behind it. Not only did this sixth outing see the return of creator James Cameron (as producer) and Sarah Connor actress Linda Hamilton for the first time since Terminator 2: Judgement Day, it also promised to retcon the other, less popular sequels out of existence. Sadly, Dark Fate was another letdown, due to a lack of originality, pointless tweaks to existing mythology and a villain that lacked any real menace.
Only Linda Hamilton and Mackenzie Davis made any real impression with Terminator: Dark Fate and planned sequels were swiftly cancelled. The sequel's opening also proved extremely controversial within fandom, as it sees a T-800 unit shoot down a young John Connor (Edward Furlong) following the events of Terminator 2. This was doubly shocking as the filmmakers managed to keep this scene (mostly) under wraps during production, but they did reveal that an older Terminator named Carl would play a role in the story somehow. While this reassured viewers that Arnie would be returning to his most famous role, it was a mistake to reveal this during the marketing phase.
For one, Terminator: Dark Fate keeps the character and Schwarzenegger's role a secret for most of its runtime. Admittedly, Carl does feature in the now-infamous opening assassination, so canny audience members probably could have guessed he'd be back, but the story itself treats it as a shock when he finally reappears. Arnie's role was used as a marketing tool, but like the reveal that John Connor was a villain in Terminator: Genisys' trailer, this is something that would have worked better if viewers had been allowed to discover it for themselves during the story.
Dark Fate ultimately held few genuine surprises, and withholding the Carl revelation could have been a big advantage. Of course, it was probably never realistic to think the producers would play coy on Schwarzenegger's involvement. Blade Runner 2049 director Denis Villeneuve once revealed he would have preferred if Harrison Ford's return as Deckard - who like Arnie's Terminator Carl, appears later on in the story - had been withheld from trailers, but understood that was unrealistic from a marketing perspective. Given how little Arnie's involvement appears to have helped Terminator: Dark Fate at the box office, perhaps a little more mystery would have aided the sequel in the long run.