SFGate

Kaplan Architects, AIA You could paint them to match the adjacent wall color to make them disappear more. I would also update the door pulls keeping the size and shape simple too.

feeny Would a built in, sliding shoji screen (or something equivalent) make sense? I'm just not a bifold door fan.

olldbobbi I totally agree with Kaplan! For texture, you could get the heavily textured paintable wallpaper and paint them, also. Long chrome handles will add a modern touch.

decoenthusiaste Another option might be to attach trim to make them more doorlike, paint the door frame white and the doors black. Add new hardware like pic #3 and they will become a stylish focal point.

inkwitch If you're interested in a more "artsy" treatment, you could apply strips of different designs of wood trim vertically or horizontally and paint the whole a glossy color of your choice. I've seen this done to the front of drawers on houzz, and plan to do it to a small nondescript chest of my own.

mpoulsom I just did something with my fireplace that turned out pretty cool, with wood strips. Not finished yet, but you can get an idea of what I'm talking about. You could remove the doors, lay them down flat and glue them on vertically instead of horizontally. It is a very modern and earthy look. Change the hardware and you would have a totally different look.

orangecamera http://www.apartmenttherapy.com/diy-wood-art-173470

This was done with wooden shims, glued on and then stained. This would add an interesting texture, which you could stain or paint.

I wouldn't recommend using tiles or anything too heavy on the doors. Too much weight will thrown them out of whack and they won't open and close properly after a while.

nevadan Wait till you can afford to do it right. The doorway needs to be narrowed and a conventional door inserted. In general a dor beside a fireplace looks bad, but yours is even worse.

Jabelone What's on the other (right) side of the fireplace? More painted wall? Paneling? Shelving? Anything too dimensional, artsy or textured on the bi-folds will draw focus away from the fireplace. You could consider the opposite approach of painting the doors to match the wall color and make them disappear.

Carolina Photo To Go LLC Can you go to a place that reclaims doors from buildings and look for French doors or the Barn Door look that slide on Pulleys.

Rustica Hardware By-pass barn door hardware could be a good option. It gives you a modern and rustic look. Not sure what your budget is be you can take a look on our site here: http://rusticahardware.com/bypass-barn-door-hardware-system/

Portlandshojiscreen.com Using handmade papers from Japan, China or anywhere, you can cover the doors. Look up Fusuma in google images or youtube to get an idea...
Real Japanese Fusuma is a shoji-like framework that is completely covered on all sides. Often they used a printed wallpaper like image for a final cover. I'm suggesting just covering the hollow-core doors with paper for a rustic style.You wouldn't need to do the inside. You could wrap the edges with new wood.
sorry, no pix!