I'm guessing Spielberg would have directed at least one movie.
I suspect that Anakin would not have been so young, had Ep 1 been made soon after Return of the Jedi. It's hard to know if Lucas would have gone for an unknown or, more likely, a little-known actor, or a star. It depends on when the movie was set and the age the character was supposed to be, but I could imagine someone like Michael Biehn as a young Anakin. Though Lucas may well have cast a Brit, given that Anakin had an English accent in Jedi.
Branagh could well have been Obi-Wan, depending on when TPM was set and whether he was looking for someone around the age McGregor was when he played the role or whether he wanted someone older (as I think should have done). Sean Connery could have been Qui-Gon or even Dooku, had the former been in an 80s prequel trilogy, though Christopher Lee could have excelled even more as Dooku when younger.
Padme - hard to know, again, depends on how old she'd have been if Lucas made the movie 15 years or so earlier than he did in reality. Jennifer Connolly? Joanne Whalley (his Willow star)?
Lucas told Terence Stamp that Chancellor Valorum was meant to be a bit like Bill Clinton during the impeachment hearings, so perhaps a pre-Clinton trilogy wouldn't have spent so much on political shenanigans (to the joy of many).
The movies would obviously have been less reliant on CGI, probably for the better (and I speak as one who likes the PT). Jar-Jar may not have appeared and if he had, he'd have been someone in a costume, like Chewie, 3PO etc. Yoda would still have been realised by a puppet and as such, wouldn't have got to fight Dooku or Palpatine. Mace Windu may have appeared, as the name had been kicking around since Lucas' early drafts, but he may not necessarily have been played by a black actor - Lucas offered that role to Sam Jackson when he said on a chatshow that he'd love to do the new SW movies.
I also think that a PT of the 1980s would have adhered to 'fanon' more than the trilogy we got did - so Boba Fett might have been an ex-stormtrooper, Uncle Owen would have been Obi-Wan's brother (and we would have heard Obi-Wan referred to as Ben) and the background would generally have been more in line with things referred to in the novelisations of the movies.