(I'm not going to change Lucy in any way. It's a pet peeve of mine when other writers change everything about Lucy so I won't do it.)
So basically Natsu betrays Lucy in a way that she absolutely can't forgive him and that leads her to 3 years in a different dimension to train. She becomes the strongest mage in all of Fiore and finds love along the way.
I do NOT own Fairy Tail, Hiro Mashima does!
Sequel: Shadowed Heart: Time Twister