[2020] If they ever do bring back Gjallarhorn, I think it should just be sold by Xur without warning or quest.

Play the nostalgic melody for us you know.

Sure a quest would be probably enjoyable, but wouldn't that "Oh sh*t!" Moment be better.

No heads up, no hints, no teases, just BOOM! Gjallarhorn! Now that would be a "I was there when" moment that no one would forget.