When Loki joins the strawhats, Imu will start freaking the fuck out because this is the absolute worst case scenario (Loki is the reincarnation of one of the world ending gods referenced in the harley text)
Imu will then call on their contract with Mihawk, forcing him to abandon the Cross Guild and return to Marie Jois to once again serve as the head of the Holy Knights. This is how Mihawk will be relevant in the endgame