I would like to see more teams, but it's limited to 12, so we'll only have 24 car grids. Of the manufacturers you name, I have the following comments to offer:
Audi: Child of parent company VW, run by ex-BMW boss, and known F1-phile Bernd Pischetsreider (sp?!!). Nowhere left to go in motorsport after dominating rallying, touring cars and GTs. Repeatedly issuing strenuous denials, which in F1 language is an admittal. Also, persistent rumours from high places regarding a Jordan tie-up/buy-out, and persistent rumours regarding a 900bhp 3L V10 engine already on the bench.
Nissan: Owned by Renault, already in F1. Have been known to produce F1-spec engines through TWR, although TWR obviously contracted to Arrows, and TWR have prior commitments/obligations to GM (see below). So Nissan not going to happen.
VW: Of the five big manufacturers (Fiat, Ford, General Motors, VW Group, DaimlerChrysler), VW is one of the two not currently participating in F1. Badge-engineers supreme, it's massively unlikely that VW would enter F1 under the VW brand. Instead, see Audi, above.
Chevy: Owned by General Motors, the only other (with VW) of the 'big five' not in F1. There is significant impulse for them to join, but I think they're looking for post-Kirch stability, and to avoid being caught in the storm of the next Concorde Agreement (due 2007) and the proposed breakaway in 2008. Personally, I feel that GM are behind the Phoenix shambles. This is because the Phoenix figurehead is Chris Nickerson, a long-time friend of Tom Walkinshaw. Additionally, TWR (Tom Walkinshaw Racing) build all the Holden 'HSV' cars, plus do extensive work for Vauxhall and Saab, all three of which are GM companies. I think you'll see GM enter F1 about 12 - 18 months after the Kirch/rights/breakaway row is sorted.
Just my $0.02. I submit it to be shot down, as always!