These are some possibilities where Luke might be.
He turned to the Dark Side.
Some theories say that in Star Wars Return Of The Jedi Luke gave into the dark side to win that fight against his father, And he wanted to finish what Darth Vader started.
Possibly Luke putting his ROBOTIC hand on R2-D2.
Or he joined Kylo Ren's little squad.
Another possibility Luke will have a Obi-Wan-Kanobi role.