I have been thinking about this. Here are my worthless thoughts (since I am usually quite wrong don't take too much notice )

Audi - good sportscar pedigree but none in F1, have already stated that Audi will not enter F1

VW - saloon car and rally pedigree is strong, alos been involved in several single seat series, but F1 doesn't sit with the 'peoples car' tag

Skoda - no

Bently - plenty of pre WW2 success, nothing since, and again F1 doesn't seem to fit the profile of a luxury car brand

Porsche - obvious choice, have won F1 titles post 1980, are a performance car/sportscar brand, biggest problem for porsche is Mercedes-Benz doing so well.

Bugatti - had a couple of F1 racers in the 50's and actually introduced the rear-engined GP car to the world championship, a few years before Cooper made it work

Seat - no

Lamborghini - have been involved in a previous incarnation of the company with no success

Suzuki - no

I would love to see VAG in F1 but I would like to see them resurrect the old Auto Union badge for F1, for two reasons. 1. The sense of history - when Merc were dominating prewar it was Auto Union who were the only marque that could beat them.
2. It would isoloate and direct the team away from the corporate monster that is VAG