There are of course several possible replacements and I'm going to outline them now.
Assuming that Holden as a brand actually survives, then the lion remains on the front of the car. I can't for instance from a marketing point of view, see Holden putting on the Camaro as that would be quite obviously a Chevrolet.
The Tenth generation Impala could very easily slip into the Holden lineup as a direct replacement for the Commodore. It comes with a 2.5 inline-4 or a 3.6L V6 which is the same broad sort of engine which currently appears in the VF Commodore.
The Malibu is currently on sale as a Holden and surprisingly comes with the same engine choices as the Impala but also has a couple of 2L engines. It is a physically smaller car but given that the VE and VF both had significant amounts of metal cut out of them to make them shorter and narrower to meet the regulations, reprofiling the Malibu for the job just doesn't seem like a difficult task.
If Holden doesn't survive as a brand, then I can see some remote possibility that the future Chevrolet Racing Team (in this case bow ties aren't cool), might want to use the Camaro but that might be a mistake considering that if you mention the word Camaro to a motor-racing fan in Aystralia, the first two words they think of are "upside" and "down".
Ford have a slightly different set of issues. As a motor company, the only real time that Ford ever put any monumental effort into motor racing was when they threw bucketloads of cash at the GT40 project. Their F1 efforts were mainly down to Cosworth; which also goes for their all-conquering Sierra touring cars; and to be honest, the coolest livery ever to appear on a race car:
It has been suggested that Ford wants to use their Mustang but that first assumes that Ford would bother to reenter V8 Supercars once the Falcon is finished. Ford in Australia dropped Alan Moffat like a sock full of custard in late 1978*; didn't really support Dick Johnson properly and have threatened to dump what's left of FPR. I can't really see Ford stumping up the cash to develop the Mustang.
The really weird thing is that Volvo are entering the 2014 season with their S60. Officially Volvo call the platform that the S60 sits on the "Volvo Y20" platform. In reality it is identical to the "Ford EUCD" platform which both the Mondeo and the Mazda 6 also sit on.
The thing about the V8 Supercars Car Of The Future program is that the platform which all cars ultimately sit upon is dimensionally identical and the specification is fixed. If Volvo can fit their S60 to it, then it should be a snap for Ford and in theory Mazda to do likewise.
Conceivably, the 2017 V8 Supercar Championship might have up to 6, 7 or even 8 brands in it. I could for instance see the Holden Malibu, Ford Mondeo, Mercedes-Benz E63, Volvo S60, Nissan Altima, Mazda 6, Kia Cadenza and Hyundai Genesis. It'd be highly unlikely but it might be interesting.
*After Moffat was dropped like a sock full of custard, he would eventually run the RX-7 at Bathurst. This caused Holden to sit up and take notice and they threw sufficiently enough money at the HDT to ensure that the plucky little RX-7 did not win Bathurst... the 12hr which came much later was a different story though... as was the Le Mans 24 Hour but that's another story entirely.