and then she is crowned as the new demon queen, the end.

She almost isekai'd her whole class

No.1 priority spell to learn - Manashield.
She'd be just about invincible then.

I wished they put a weakness,

Example, She doesn't have enough magic power in her body to fire powerful magic, like the one she just did, but she still feels fine, that's because it used time off of her life, so every time she uses advanced magic it works but pulls her closer to the edge. then when she is feeling like magic is her everything the sage comes out and explains this to her and tells her she only has a few years to live. so she picks up the sword and follows her original dream of becoming a swordswoman, then it can either end with her finding a way to bring back her lifetime, or just with her saying farewell and thanking the sage and teachers for teaching her about the wonders of magic.

Just thought up of this on the spot. The story probably wont do this but its a nice fan fic.

and than she reincarnate to in other dimension Nanoha x"D

Soon you'll be megumin Dx