You could do something like a craftsman style thingy (it'd be WAY easy to weld something up yourself, they liked clean straight lines with that style, dunno if it'd work for your room, though).
The biggest problem, which this broad (sorry,
pocket-emptier decorator) doesn't seem to be seeing is that you'll have to get a screen that BLACKS OUT the empty unused hole, otherwise, every time you run the fireplace, you'll notice the big fat empty hole, and it'll grate on you like nails on a chalkboard.
Fix the fireplace. Center it. They may have to rebuild your stack, flue, whatever it's called, but you can also at that time de-collossolize the sheer hulking mass of that fireplace (or leave it, after all, it's your house, and I'm sure whatever you decide to do will look good from my house

) and center your fireplace, or just center it and make the stuff look "right".
That big friggen hole isn't something you can "hide" with a screen, since the fireplace itself is off center.
Is your OCD making your hand shake whenever you look at it? Is that tic making it hard to see straight? Glad to have pressed every floor on your elevator!
