I think she'd work best in X5 with the OT cast. This may be a bit cheesy, but this is how I imagine Psylocke being introduced:
One of the Ppening Scenes for X5:
Film opening scene. It's the future again. This time it opens at a mansion with high-tech security. An intruder manages to slip in, entirely disguised and apparently male. The outfit seems to be armored, but light, and ressembles something like this, with lighter shoulder pads and a violet tone:
maybe a bit like this, but definitely genderless:
The suit's technology helps the intruder deactivate key alarms, unrecognized, but is equipped with key compartments, some of them filled with ninja-esque gear. The canon is used to disable guards, shooting violet knife-sized bullet beams, apparently generated by the canon itself.
Inside, the intruder manages to retrieve something from a vault and hones in for a final stroke: the kill.
Mr. Worthington Sr. is unaware. He steps out onto the balcony in the moonlight, looking up to the night sky. A newspaper at a table hints mutant connflicts and a politica controversy involving the Worthingtons, next to a personal photograph of a young and handsome blond man.
A tall man in a coat walks on grounds. He realizes the guards have been disabled but they are physically unharmed. He looks up at the mansion.
Back on the balcony, the futuristic assassin scales the wall silently, eyes wide and fixed on the target. She fixes the canon, charges, and
SWOOSH!
A winged creature takes off with the intruder and drops him on ground. This breaks the canon, revealing that the intruder's knives are not mechanical but mutant-powered.
The winged creature swooshes through the sky and back down and we realize that it is literally an angel - a blonde, handsome and muscular winged man (
new cast member, please). The intruder generates a violet blade and attempts to stun the intruder with it, but he dodges swiftly. He catches up to the intruder and struggles with him, talks with him to no response, but the intruder pulls up a real kunai knife and stabs him in the chest, this time with real physical pain.
Angel falters a bit and the intruder flees. He quickly flings the knife back at the assassin and, hitting a tree, the intruder escapes.
Mr. Worthington, having watched the altercation, runs back to the vault and realizes what has happened, and murmurs
El-Saba-Nur with dread.
Angel walks closer to the tree and realizes that a lock of glossy black-violet hair was pinned to the bark, hair that was freed from the intruder's helmet because of the drop from the balcony, next to a violet-blue butterfly on the tree. He takes the lock of hair and the buttefly flutters away. He looks up at it and realizes the helicopter lights and sirens are here. A swoosh of wings.
End scene.
Another early scene:
Angel, Rogue and Kitty, out shopping at NYC. Mr. Worthington Sr. would be giving a public speech later that day, and they were there just to ensure his safety.
Angel recognizes a sketchy individual in coat, scarf, glasses and hat, but he recognizes
the violet hair. He takes off and leaves Rogue and Kitty behind, not realizing that Gambit has been watching them for the moment to take Rogue, whom Apocalypse has chosen as a Horse(wo)man.
Angel chases after the disguised mysteryman. After a lengthy, laborious chase, Angel finally takes off the disguise and sees Psylocke for who she is, a woman. He's mesmerized. They have a brief conversation. She explains her mission to stop Apocalypse from being released from confinement, to take a special key from Mr. Worthington Sr. and ensure that he took the secret of the keys to his grave. However, there is a second key that one of Mr. Worthington's colleagues knows of, and he is present at the same event. She then points out key enemies in their surrounding, two of which are current Horsemen. Because everything appears to be at peace for the time being, they pause. A butterfly flies between them briefly and she explains its significance. It represents inevitable change and growth and how one must learn to adapt to it with what they've got. There is a brief romantic pause.
Kitty then screams for Angel's help. There is a turmoil in the crowd below. He leaves Psylocke and realizes that, up above, from the tallest scraper, his father and a few other men were being held over the air while a group of mutants terrorized the audience below. The men are dropped. He dashes. Like a race against time, Angel realizes his wings aren't fast enough. A few men point their guns at Warren, but Psylocke disables them from afar.
Warren catches his father last second, wings breaking against the concrete steps below the scraper, bodies falling around him. The crowd dissipates. Angel's father is dead from the impact in Angel's arms. And Angel is left on the ground, twitching. Kitty runs up to him. Rogue is gone. Psylocke engages the one Horseman left on scene and its remaining supporters, and is aided by latecoming Wolverine & the rest of the X-men. Psylocke takes down the Horseman and the X-Men protect the civilians from the ensuing destruction & the rest of the terrorists.
Summary of later scenes:
Psylocke joins the X-Men for her mission. She falls in love with Warren, whom struggles due to his wing loss and his father's death, but she's unable to help him and her training as an assassin is not enough to stop her from hurting.
During his ressentment, uselessness and guilt, Warren is approached by Apocalypse. He disappears before the climactic scenes of the film unfold. Psylocke is concerned about his whereabouts but has no time to search for him, their final chance to take down Apocalypse has come. Little does she know, she will face one of Apocalypses new horseman during this final encounter, and it is none other than Archangel, whom, rather than adapting to a new change in his life, was seduced by Apocalypse to become one of the X-Men's fallen angels.
(I apologize for the length. I know I personally hate reading endless posts, but I got carried away.)