Thing was, Burton did not even want to direct Batman Returns in the first place. He did it because the studio basically told him: "Think of it as a Burton film, not a Batman film." And then they were surprised when Batman Returns ended up being closer to Burton's other works than Batman 1989. So, I'm not entirely sure Burton would have been all that interested to direct come the sequel.
Burton did read at least a few comics, I'm pretty sure of it. Specifically I think he read the Killing Joke. That particular comic informs, essentially the entire story of Batman 1989. And other sites have detailed how much of his work is lifted from comics from 1939. For all I know, he may still be generally disinclined towards comics, but the man did his homework, I'll give him that.
The film would not be called Batman Forever, Burton hated that title.
I think Robin would still appear in the third movie, the studio pressure would just be too big at that point. They've waited two films for it now. But I somehow doubt they'd cast Chris O'Donnell in the role. Hell, Burton might go ahead with the BR plan to cast Marlon Wayans in that role. And no I'm not trying to be funny here, Marlon Wayans was actually cast in that role. They just couldn't fit it into Batman Returns.
Billy Dee Williams is not coming back. There's a contractual thing going on there from what I've heard. Essentially he was supposed to be in Batman Returns, when he wasn't the studio had to pay him for a breach of contract. They would have to pay again to get him in the movie. Not happening.
Thing is, Burton might care a little more about internal consistency in his Harvey Dent. Even if he won't hire Billy Dee Williams, he might get a black actor to play the part. Don't know who it'd be though.
Williams was still (I think he still is actually) sore about the whole "being used as bait for Jack Nicholson" from 1989. He's probably not going to be on board either.
I think Michael Keaton was getting sick of the role by the time Batman Returns came out. It really comes across in interviews he did at the time. That said, considering he agreed to do Batman Forever quitting early on in the production. So he'd probably be willing to do one more movie.
Batman Returns implies that Selina Kyle was still out there, so she might cameo.
There's a good chance the planned Catwoman spinoff actually happens and it doesn't turn into the movie we know as Catwoman
I haven't the vaguest idea what the plot would be, it's tough to know what he was planning when he left the directors chair so early in the process. There's a chance the "Riddler with a pet rat" idea ends up in the movie. I'm not sure if Two-Face will. But having set him up in Batman 1989, I'd think Burton would want to use him. But you never know.
Oh and if the script is a decampified version of the one we know, Renee Russo is Chase Meridian.