Please read the description first.
The protagonist, Chise, is Chise from The Ancient Magus' Bride. I haven't read the manga, and I'll be combining elements from other anime. It may be a bit of a Mary Sue story, so please be aware of possible out-of-character portrayals.
Warning: In this setting, Chise's world is a parallel world to The Ancient Magus' Bride. Because of a girl who transmigrated into this world, Chise and Elias, her canon pairing in the anime, never got together and aren't close. In the end, when Chise tries to save the transmigrated girl, Elias experiments on her and sends her out as a decoy.
Everything happens because the plot requires it. Everything happens because the plot requires it. Everything happens because the plot requires it. Important things should be said three times! I do like Elias's long legs and bones, but liking them and putting them through angst aren't mutually exclusive (trying to convince myself).
Also, the transmigrated girl isn't actually bad. In my view, not every girl who transmigrates has to be a villain. It's not the girl's fault that Elias tries to save her. Elias is simply that kind of person (whispering). In the original anime, Elias wanted to trade the life of an innocent girl for Chise's, but Chise stopped him. In the parallel world, Chise is no longer the most important person to Elias, so naturally her standing is about the same as that innocent girl's.
Well... I think those are probably all the warnings. If you really hate breaking up canon couples, please be cautious, cautious, cautious! If there are other things you can't handle, there's nothing I can do. If you really can't keep reading, just click the dislike button in the upper right. I'm not good at arguing, and I don't want to be personally attacked.