If by chance Ahsoka managed to escape Order 66, The Clone Wars series could do a story arc set in the immediate aftermath of Episode III where Vader is doing the initial purge of any Jedi suspected or rumored to have escaped the Clones assassinations. Darth Vader in his suit in the Clone Wars animated style would be awesome to see and would also create tons of merchandising opportunities for the franchise.
This would be a very dark way to end the series, but not the darkest. Just wait till I get to the end of the list.