The Mad Titan will be sporting a different look when he begins gathering the Infinity Stones in Avengers: Infinity War.
The change was noted when the San Diego Comic Con 2017 poster for Avengers: Infinity War had Thanos front and center, but without his helmet and armor.
One instance of this came at the end of Infinity Gauntlet when Thanos embarked on a much different spiritual journey.
Since the Infinity Stones give the user omnipotence, Thanos honestly wouldn’t need armor.