Could Morgoth come back?

[Rimbaud]

Melkor indeed does return to the circles of the world. He is 'at large' for the Dagor Dagorath, the final battle of Tolkien's mythology. To steal a quote used by our fellow forum poster Legalos,

Manwë will not descend from the Mountain until Dagor Dagorath, and the coming of the End, when Melkor returns.

...and this is all contained within the Silmarillion, which is a very good place to start in search of answers for such questions.

With all this talk of Doomsday, arm-a-geddin' out of here...