hard to tell what color it is painted. looks white in the picture. perhaps paint it the same color as tiles (one of the colors) maybe a darker color.

Or continue with like tiles to the top.

Maybe pull the tiles off the face and continue the plaster all the way up to the ceiling.

HI -- maybe if you continued the white box part all the way up to the ceiling and then add a wood beam / wood shelf at about the 40 to 42 inch height to the width . Then paint any of the white showing below mantel to a black/dark brown . Then mount a fancy round mirror or a décor platter above mantel . Or reface the whole front to the ceiling with a new tile or full sheet of stone etc. after re-moving the old tiles . Then a beam mantel board can be added or not .

Is the wall part of the mantel just sheet rock behind it? If not bring it to the tile height and top with a varnished dark wood piece of wood. That will make it pop, then paint that wall a soft tan color as an accent wall, make sure to have crisp lines at the corner

Just an idea but...Hire a painter that does faux painting. Have him/her paint the cream part above the tile to look like the current tile.

You can absolutely make it wider. It is fairly inexpensive to relocate an ac duct if you are on a raised foundation. If these are cast in the slab have a 90 made so it comes out the side of the fireplace.

Hard to fix the proportions otherwise as you are aware.