We've seen a few things from the clone wars show up here and again in Rebels. I think if the producers wanted to make a point of showing an old, run-down Imperial cruiser of some sort, they'll haul out a Venator or Acclaimator to feature in Rebels. In fact the Acclaimator may still have a role in the Empire because it's the largest ship that can land directly on a planet and disgorge troops. If Rebels ever needed to depict a full-on planetary invasion, they'll show those Accs landing and releasing AT-ATs and full troop battalions.
I think a Venator showed up in a cameo appearance in Rebels, with Tarkin having a model behind his desk. While VSDs are technically cannon via the Tarkin novel, in reality the oldest on-screen Star Destroyer is the Venator.
