There are a couple of things you can do to make it look like that thing in the corner was intentional.

If you'd like to modernize it, the river rock probably needs to go. Carolyn offered some great suggestions. I like how a fireplace looks floated off the floor, which eliminates the need for a hearth extension at the floor level (unnecessary for most gas fireplaces, anyway).

However, the down side is that floating the fireplace forces the TV to go up higher. So in your case, I'd suggest leaving it on the floor. And I'd suggest installing a hearth extension in front of the fireplace to make it look right.

If you decide to do that, it would be best to recess it into the floor so as not to interfere with the access door at the bottom of your fireplace.

Frankly, If you can swing it, I'd really recommend upgrading the old H&G fireplace to one of the newer, more realistic models before spending a lot of money on the wall, but I don't know how motivated you are to do that. : )

If you'd like to "future proof" your project, I'd suggest filling in the hole and just wiring all of your connections to the wall face. Use a universal TV mount that will allow you a few options on you size & model TV. Relocate all of your boxes (cable, DVR, DVD) to another nearby location and use a universal on-screen remote system to control the lot. Getting rid of the obvious technology will help update the wall more than anything else.