New Title - Rebirth.
Gregor (now an attractive, enigmatic high school student prior to undergoing the metamorphosis) no longer transforms entirely into a giant insect, instead he just develops certain insect-like characteristics. These could include the ability to walk on walls, a heightened sense of smell and superhuman strength. Also, two pairs of large, foldable wings grow from his back, enabling Gregor to fly if he first takes off his shirt.
Grete, Gregor's only real human contact for the majority of the story, is no longer his sister. She is now an attractive, aloof girl from Gregor's English class who has always harboured a crush on him and, after noticing that he has mysteriously missed several weeks of school, goes to his house to investigate.
The story should be told from Grete's perspective.
Need a reason for most of the action to take place at night. Should Gregor be nocturnal? No, too limiting. Maybe Grete sneaks out of her house each night to visit Gregor in order to keep their burgeoning romance a secret from her strict, overprotective father?
Change the location to a small coastal town at the edge of a forest, where it is perpetually foggy and/or snowing.
Should we explain the reason for Gregor's metamorphosis? Result of an ancient curse cast on him in an attempt to teach him some important life lesson? He is descended from a long line of shape-shifters that each metamorphose into an insect-human hybrid upon reaching the age of sixteen? Yeah, let's go with that.
Also give Gregor a trio of older, more boisterous, half-insect brothers.
A tribe of arachnid people, the sworn enemies of the insect people (think of a backstory later), move into town, sparking a feud between the families.
Central conflict - Grete finds herself torn between two romantic interests: the considerate, introspective Gregor and a more exciting, free-spirited arachnid person.
Try to come up with better names for these creatures than "insect people" and "arachnid people" (look up some Latin biological terms for ideas).
If you decide to make Gregor's metamorphosis symbolise puberty also give him mood swings, a strong sense of anxiety about his changing body and a newfound rebellious attitude towards authority figures.
Idea for a cover illustration - Deep within the forest, a giant, recently opened cocoon hangs off of a beech tree branch, illuminated by the moonlight. In the background is a silhouette of Gregor, shirtless and soaring through the night sky.
Possible fonts (for the main text): Constantia, Century Gothic, Franklin Gothic Book, Perpetua.