What if one changed the way Orihime Inoue powers worked? Her rejection could be imposing her own version of reality. In canon is said that the only real limit to her powers is what she things is possible. What if that, when Byakuya Kuchiki "Killed" Ichigo the first time, destroying his shinigami powers. Orihime was the first one to find Ichigo? She of course would have tried to heal him, since he was really injured. Now the idea is that since Orihime was desperate to save Ichigo life, she left a heavy imprint in Ichigo spirit, basically changing it. While that would not have been enough to leave any everlasting changes, when Ichigo regained his soul reaper powers, that imprint fused with Ichigo shinigami and Hollow forms...
Things seem to have gone as planed, Ichigo is again a shinigami but... there has been an important change.
Ichigo is now a girl, while Ichigo could fix it, since "She" has not returned to her human body yet, doing so would mean losing the shinigami powers permanently. Being Ichigo, even the treat of being stuck as a girl version of himself with the “assets” of Orihime is not gonna stop her from trying to save Rukia.
Also, not everything is wrong, the soul society is not expecting "her", and both Yoruichi and certain golem girl aren't gonna miss the chance to instruct their new "sister" to to infiltrate soul society. While at the same time Ichigo friends try a parallel rescue attempt to distract the Gotei 13.
How?
The idea is that Orihime reality powers work by imposing her own reality over the TRUE reality. After Byakuya Kuchiki defeated Ichigo, it was the point when Ichigo spirit was as its weakest, Orihime got her powers by being around Ichigo, so her power had a similar enough signature to change Ichigo without much resistance. And since back then Orihime was just using her powers as an instinctual level, and she only wanted Ichigo to live, her will imposed over Ichigo own. Ichigo had a crushing defeat, he not only lost a battle and his powers, but also Rukia, so he barely offered any mental resistance. Last, because Orihime had no clue how her powers worked, the result was her tainting Ichigo “Spiritual Matrix” with Orihime own.
"What changed you was a series of incredible circumstances and coincidences that only happen in badly written fantasy stories. Of course. the fact that one of your friends can warp reality didn't hurt to make it possible."