I have always liked the text and traditional tune for “The King Shall Come.” The linked recording with the simple GIA arrangement is a bit slow for my taste. I’m not in favor of the Picardy drama at the end, not for Advent. Alter the harmony if one must, but keep it in F minor, E minor, G minor, or whatever the chosen tonality might be. As a shape note tune, it translates a bit better for guitar than organ, I think. Also, quite sturdy and singable a cappella.
Trevor Thomson has a creditable contemporary setting of the text. The people at my previous parish advocated for it, but we never got to implement.
Not every parish I’ve served had the traditional tune in the repertoire, but most did. I wonder how widespread it is these days. Many folks, especially monastics, prefer “Creator of the Stars of Night” as the lead-out Advent hymn. Modern Catholics seem less impressed with that tune. It would be my choice, but my choices don’t always sit well with folks in the pews.
Thoughts?
Catholic Sensibility 
IF you don’t want an explicit Picardy third, another option is simply to *omit* the third, and let the ear and acoustic resolve the modality … perfectly. It’s an old trick, and a good one.
“Modern Catholics seem less impressed with that tune. ”
Btw, I am curious for the basis for that observation. I’ve never witnessed the tune not be engaged well, and that’s across multiple communities of different types. Yea, it’s not Holy God We Praise Thy Name, but it’s pretty solidly entrenched as best I can tell.