When Kylo talks to Vader’s mask
In The Force Awakens, we get our first glimpse of Vader’s melted helmet from his pyre on the Forest Moon of Endor. Kylo Ren is speaking to it, trying to draw Dark Side energy from it as he questions his commitment to the Dark Side.
This would’ve been the perfect opportunity for Hayden Christensen to return to his role as Anakin Skywalker. He could’ve appeared as a Force ghost and told Kylo the path he’s chosen was the wrong one, and try to convince him to go back to the light. He could’ve said how Darth Vader always had light in him, and never fully turned to the Dark Side.
This also could’ve been a great opportunity for an interesting subplot. We could’ve seen the Force ghost of Anakin trying to communicate with Kylo Ren but unable to reach him. He is aware that there’s something blocking him from communicating with his grandson. He could’ve relayed his concerns to the Force ghost of Obi-Wan and they could’ve had moments throughout the trilogy where they try to figure out this source and eventually come to the realization that it’s Palpatine who is blocking Ben from communing with their Force spirits.
As good as the Star Wars sequel trilogy is, it’s these missed opportunities that make it a frustrating series of films.