I imagine Chrysler will enter under the Dodge brand using the Charger body or possibly the 300C Chrysler body.
They have said that if they were join, they would use the 300C.
Hyundai would probably use the i45 or maybe the Elantra? I think more likely the former.
I’ve heard the i45 is the most likely choice.
Audi would almost certainly enter using the A4.
I’d say they would be more likely to use the A8, which has a V8 engine. Although having a V8 engine is not a prerequisite of the base model of car, Audi would probably want to use one with a V8 engine to begin with for consistency.
I cannot think what Nissan would use, it would have to be the Maxima based on their current range unless they bring out a 4 door GTR next year.
Pundits are tipping the Nissan Altima, which will replace the Maxima some time this year or in 2013. As far as I know, there are no plans for a four-door GT-R. And I think it would be a very poor choice; the GT-R is the spirtual successor to the Skylines, and the last time Nissan was in V8 Supercars (then known at the Australian Touring Car Championship), they used the R32 – and it didn’t end well.
Infiniti don’t really offer a lot of choice either, with it basically being a fancier Maxima.
If Nissan were to enter with Infiniti, then they would probably do so with something from the M range, probably the M37.