Captain Cold (Wenworth Miller) is returning to The Flash in 2018 and could return again in the future, assuming the story is right.
It will presumably be the heroic Citizen Cold who shows up on The Flash next year, paving the way for a small crossover between The Flash and Legends of Tomorrow in the process.
First though, he will be making (at least) one more pit stop in Central City to visit his old buddy The Flash.
Not too long ago, Miller indicated that he will be leaving the Arrowverse, signaling the end of his time as the many faces of Captain Cold.