The true successor to the Empire:
Grand-Admiral Thrawn.
Though it seems Disney would not accept that a capable military leader would rule the Empire.
In Return of the Jedi, a single battle was lost, the Empire lost many of its best ships and officers but they still controlled a vast chunk of territory and a massive Imperial Fleet.
The books by the Timothy Zahn should have been the next Star Wars movies, as the writer tried to present a 'realistic' situation to the continuing war. He also created great storylines for all the main characters.