The official Expanded format is what Lurtzy described. All sets legal, following the X/R-lists above, and sites only from set 11 (Shadows) on.
Another (unofficial) approach is, as Elrohir described, to scale back Standard format to before Towers Block was rotated out and Hunters was released. For "authenticity" that would be sites from set 11 on, and following whatever X/R-list was in use at the time. Maybe you could borrow later sites, or both use the RotK site path (possibly adjusting the X-list for broken combos).