The X-Men's Kitty Pryde once cosplayed as Darth Vader. Back in the 1970s, Marvel Comics acquired the license to Star Wars, and they began a popular and beloved run of adventures that are sadly no longer considered canon. Still, surprisingly enough, they left their mark on the X-Men franchise - after a fashion.
Naturally, Lucasfilm was wary of allowing any kind of crossover; after all, they didn't want to risk permanently binding their continuity to the Marvel Universe. But that didn't stop writers and artists taking another approach; they began dropping explicit references to the Star Wars franchise in their comics. In fact, on a handful of occasions Kitty Pryde - the X-Men's youngest member, and a reader surrogate in the team - was shown reading Marvel's Star Wars books.
Matters came to a head in 1982's Uncanny X-Men #155, when legendary X-Men writer Chris Claremont took the X-Men on one of their infrequent cosmic escapades. Lilandra Neramani, Majestrex of the Shi'ar and lover of Charles Xavier, had gone missing; the Shi'ar pursued her kidnappers to Earth, and were prepared to level the planet in their attempt to find her. Xavier negotiated a single day's grace for the X-Men to track Lilandra down, with two mutants left behind as surety. One was Kitty Pryde, who had been teleported up to space while in a swimsuit; she soon availed herself of Shi'ar technology to create a new costume, trying out a number of amusing looks.
The costume-making machine is clearly analogous to the one introduced by Jim Shooter in the Secret Wars miniseries, which gave Spider-Man his symbiote costume. In this case, though, Kitty is only clad in normal fabrics - and artist Dave Cockrum quite enjoyed himself. One of the costumes was actually an alternate design for Phoenix - it even bears the distinctive emblem - but notice which costume Kitty considers "sublime." Like any fan, she uses alien technology for cosplay; she's suddenly been dropped into a sci-fi adventure, and she figures she should take advantage of it.
Claremont is a master storyteller, and as such this device - originally seen as just a joke - proved essential to the plot. With the one-day deadline expiring, Kitty Pryde and Nightcrawler realized Lilandra's kidnapping was actually a conspiracy to destroy the Earth. They delayed a rogue Shi'ar operative by having Kitty dress up as Dark Phoenix, a perfect distraction; Kitty used her phasing powers to lead them on a merry chase, while the rest of the X-Men arrived to save the world.