Your pic didn't post.

It was probably meant to be a display niche. I've got a space similar (but larger) to that above my front door, and I displayed some very large African baskets and Navajo rugs in that area. (Was a major pain to get stuff up there on a ladder though.)

Not a Pro.

I am a true believer of putting nothing in the space, but I would accent it to give it interest.

On the back wall, I would do one of the following, mirror, paint an accent color or wallpaper and be done with it :-)

I'd prob paint that back wall a darker color, or do some type of wall trim treatment.

hang a large picture, or do some tall vases of some sort. up to you what you like

you could always look into closing up the gap and extending your loft to include a little sitting area

Thank god those plant ledges stopped being included in designs. I say leave it blank. Anything you put up there will just become a dust collector