I think the first thing the American companies need to do is beat their reputation for having bad quality control and countless problems. An example of a car that was otherwise competitive with the imports but had its reputation ruined anyway is the Ford Focus. Some may not like it but it's a reasonably valid alternative to the other econosedans out there. The problem is that it had something like 10 recalls in its first year and the public made the conclusion that it was a piece of crap.
All of the Big Three have the capability to put out some much better cars than the crap they get away with in the US market. Some of these are models in their lineups for other countries that would be totally feasible to sell here.
Ford Europe has the Mondeo, Ford Australia has the Falcon (a rwd muscle sedan kind of like the Holden Commodore). Sell those instead of the Taurus and the Crown Vic and it's a decent lineup.
The Vauxhall/Opel Astra would make a great econo car for the USA. The new Vectra would be a fine competitor for the Accords and Camrys of the world. Plus there's all the assorted Holden models. The next Commodore could very easily be rebadged as both a Chevy Impala and Pontiac Bonneville. The new Chevy Malibu is based on a new GM global platform but it still manages to be boring as all hell. They could have at least made it more like the Vectra in appearance.
Chrysler doesn't really have much of an international presence and Mitsubishi models aren't exactly a great inspiration, but the premium rwd sedans with Mercedes parts are definitely a step in the right direction. Still, I think the 300C is too foo foo lookin and the Magnum should have a sedan version.
I think in general the American companies need to have a quality fwd econo sedan and mid sized sedan, plus a step-up rwd sedan and coupe. The only car right now that tries to compete in the step-up-from-family sedan segment is the Nissan Maxima and it's a bit of a fwd boat. The Big Three could bust that segment wide open with cars like the Commodore and Falcon. For coupes, Ford could bring back the T-Bird as a genuine big muscle coupe to compete with the GTO. Chevy could perhaps tap the GTO/Commodore when they are redone in 2007 for a Monte Carlo replacement.
Damn, I should get hired in Detroit.