As you know, once Singapore Airlines officially ceases nonstop operation to LAX, and EWR from SIN with their Airbus A340-500s, Qantas' SYD-DFW will the longest nonstop flight. But then, I realized that Qantas will be retiring their Boeing 747-400ERs by 2018, which the route is operated by. I honestly do not know the pax load of the roundtrip flight, but if it is popular, I bet they'd put the Airbus A380 in its place once retired. The A380 would have improved maximum take-off weight, thus providing a better payload/range performance by giving it an additional 190 km (100 nmi) in range.
When it comes to 2018, will Qantas continue operating to DFW with the improved A380 they plan on getting? Or unless the route isn't popular, will they cease nonstop operation like Singapore Airlines will be doing with LAX and EWR? Another idea might be using the Airbus A330-200 or -300 and make it via LAX?
Right now, I don't know if they will stop the YSSY-KDFW-YBBN-YSSY route, mainly because of one thing: OneWorld. They pulled out of SFO pretty much so they could be in bed with their OneWorld partner in AAL. The question then becomes if this new A380 will have the same range, if not longer range than the B744ERs they are retiring, or if they will go with something else altogether (read: B787-10X, A350-9000/1000, etc.). We know that the B787 *might* have the range (COA/UAL were thinking it with their KIAH-NZAA run), so that could be an option.
Another thing to think about is if KDFW will be able to service the A380. If they can't accommodate, the A380 then is out of the question, and if QFA still wants to use it, then LAX, SFO, or IAH would be their options.
A third thing to think about: AAL/AWE merger. That could bring KPHX into play for this "world's longest route". Phoenix would still remain headquarters for AWE/AAL. If they wanted to do that, they could have both the longest and second longest route there, and bring the A380 back into play. Question there is if the A380 will fit on the bridge.
Right now, way too many unknowns here to guess. But there are a number of options.