Gregor the Overlander and the Return of The Warrior

I love the Gregor the Overlander series, but like many wasn't satisfied with the ending. This is effectively an epilogue of Gregor's eventual return to the Underland.
Four years have passed since Gregor slayed the Bane and left the Underland. But the memories of his adventures haunt him. Gregor has moved to Virginia and longs to be back in the Underland. He feels out of place in the Overland now. He wants to be back flying on bats and training in the arena. Most of all, he wants to be back with Luxa, and he might just get get that chance.

This is also going to be set in 2020, I know the books were set in the early 2000's but it is to hard to write from a time where I was really young and don't remember so...