Long time reader, first time poster – so be gentle!
If we are going to see more manufacturers, it would have to be the right brand.
Lots of people have said VW, but I don’t think that’s a good fit for their image. You don’t look at a Golfs, Polos or Up!s and think “Hmm, sporty” – they are a ‘safe’, aspirational brand – F1 would not add to it.
Porsche – I have always felt is better connected with Sportscars, Le Mans specifically.
Would like to see Porsche take over at Le Mans with Audi heading back to rallies, but I can’t see that. I think Audi would be a good fit for GT racing, but that seems to be in a permanent state of flux.
So that leaves Bentley (not an F1 Bentley, god please!) or, in my eye the most likely, Lamborghini. They were in it before, even had an abortive stab at their own team (1991) and would be an interesting rival for Ferrari…
Speaking of rivals for Ferrari, Alfa have the history, but not the cars (any more). Now all about style, I don’t see them coming to racing at all (in any works capacity anyway).
Further afield, Honda have to come back, would like to see one of the Big Three from the US put in an appearance (would prefer Ford, but any would do) and a couple of smaller independent/privateer teams (making the step up from GP2 maybe?).
I know all this would lead to 30 car entries – but that has happened before. And it was fun.