As I understand it only a handful of new episodes were completed at the time the cancellation announcement was made. They were nowhere near a complete season's worth of stories....not even close.
Well, what we know we are getting for stories is:
-A story about a botched Clone going "66" (in the HD clip widely circulating)
-The return to "Clovis" Arc
-Something involving Gregor (the only redeeming feature of that god awful Gaston arc)
Assuming that's a minimal arc of 3 episodes and the possibility that Gregor is in one of the other two arcs... that's 6 episodes minimum and 12 episodes max. I'd say they have a large chunk of what was to be season 6 mapped out.
There was also IIRC, Filoni mused about explaining Maul's survival from being cut in two.
The only thing I feel cheated out of is seeing Ahsoka and Ventress teamed up long term... but if that happens to be a spin off series that would be kind of radical.