Scenario: Joker wants to take over the empire.
Joker has Palpatine's underground support via weaponry, manpower (Clone Troopers), and transport vessels. Joker also has access to his own regular resources.
Darth Vader, of course has his regular resources to tap on.
Joker and Darth Vader starts off on different planets, both are aware of each other's location. Darth Vader treats the Joker's forces as part of the rebels, nothing special to him. Both of them have 1 year prep.
Both sides in-character, winner by having opponent dead or incapacitated long enough to be captured.