I think any movie adaptation of Follies is doomed to fail.
We'd be far, far better off if Craig Zadan and Neil Meron did a television version of it in an actual old theater.
They could get any of a number of 50-something performers for the four leads (including Christopher Walken for Buddy) and some old broads for the specialty acts. They could have cameras all over the theater and tape the book scenes in the lobbies and lounges and the pastiche numbers on the stage.
Since they wouldn't have to construct sets, perhaps they could spend the set budget on Loveland, reconstructing Boris Aronson's Loveland designs.
But I worry for what unimaginative ghost effects any film version would feel compelled to create.