Dave Filoni hints that a major villain is going to join Rebels in the final season and the Noghri from Legends EU will be appearing as well as Rebels keeps blurring the line between new EU and Legends EU.
http://www.nydailynews.com/entertain...icle-1.3576692
so who are the villains that we could see.
Star Wars - Vader, Tarkin - we've already seen them both
Empire Strikes back - Vader, Palpatine, Bobba Fett - Its unlikely that Palpatine would play a major role in rebels as he's left the Jedi Hunt to Vader/ Bobba Fett could be interesting as he also had a prevalent role in the Clone Wars
ROTJ - Vader, Palpatine
Phantom Menace - same as above
Attack of the Clones - same as above
revenge of the Sith - Same as above
Rogue one - Krennec, we already know he's showing up.
Clone Wars - Barris - The fallen Jedi who when we saw her left had betrayed the Jedi due to dis-illusionment.
Out of the major villains I can only see two that would have a major impact. Bobba Fett, who could be hired by Thrawn to hunt down the Jedi, or hired by Vader. Or Barris as the final Inquisitor, but she's not really a major Villain.
We know that Ventress is dead, they killed her off in the new EU and in the legends EU, but she would have been interesting as she'd finally been redeemed.