Han Solo (Harrison Ford) returned in Star Wars movies.
In doing so, it had to complete the arcs of several characters, with Rey (Daisy Ridley) defeating Emperor Palpatine (Ian McDiarmid) once and for all, with some help from the redeemed Ben Solo, who shed his Kylo Ren persona and came back to the light side before eventually dying to help save Rey. Kylo Ren's redemption was always likely to happen in The Rise of Skywalker, because it both rhymes with Return of the Jedi and fits with Star Wars' broader themes, but how it went down could've been improved with one character switch.
Although seeing Han Solo again was always going to be a crowd-pleasing moment, the way it happens means it doesn't fully work in the moment. Han doesn't have the Force, so it's not fully clear exactly how he appears, with it only later apparent that Star Wars: The Force Awakens, utilizing the same dialogue but giving it a new meaning, but like much of The Rise of Skywalker the moment doesn't quite land with the emotional weight necessary, not least because of it's somewhat confusing presentation. To help fix that, and a greater sequel trilogy problem at the same time, then the return of Christensen's Anakin in Force Ghost form might've worked better.
The sequel trilogy badly treated Anakin's legacy, to the extent that the movies never even used his name. Instead, his redemption fell by the wayside to focus on Kylo Ren as a Darth Vader fanboy; he wanted nothing more than to emulate his grandfather, the only member of his family he looked up to after being seduced to the dark side. Kylo Ren may have killed Han Solo, but he's much more defined by his position as the would-be new Vader and his struggles to fill that void, and so it'd make even more sense to have Anakin be the one who appears to him. With Vader's mask already destroyed, this would then completely shatter the idea Kylo has of who his grandfather was, revealing the true, good Anakin to him in the moment he needs to most.
Anakin appearing here as a Force Ghost fits with the established Star Wars lore, and it could've been powerful to see the redeemed Darth Vader go on to bring back his grandson in turn, speaking to the power of redemption, family, and the Force itself in the saga in a way that the Han scene doesn't quite manage. It would also mirror the Force Ghost scene between Rey and Luke (Mark Hamill) in the movie; as the latter convinces one young Jedi that her lineage as a Palpatine isn't important, his own father could convince Ben Solo to truly let the past die, forget about Kylo Ren, and return to the light - which would also mean Anakin helping to defeat the dark side and bring balance to the Force once again.
It also would've simply been great to have Christensen return to the fold. The actor was slated for his performance in the prequels, but Star Wars fans have since forgiven Christensen and he's been a firm favorite at events such as Star Wars Celebration, but sadly that never extended to the sequel trilogy. This was the final movie in the saga to which Anakin gave his name, and having his Force Ghost appear in this pivotal scene was one of the few opportunities he could've fit in (the other obvious contender being during the Jedi voice sequence on Exegol, though that wouldn't have been a standalone moment just for him).
It would've given yet another layer to the title of The Rise of Skywalker, and brought more depth to Kylo Ren's arc as he was truly forced to change everything he believed in. Han was given a great farewell in The Force Awakens, and a touching scene in Star Wars 10 (if it ever happens), but there may not be a greater thematic chance to have him return.
Star Wars: The Rise of Skywalker's Ending Explained (& What Happens Next)
