What Expanded Universe / Legends Star Wars characters can we hope to join the TV show Star Wars Rebels in season 3 and beyond ?
Some of the Dark Side characters that could possibly join Rebels after the introduction of Malachor V in the season 2 finale twilight of the apprentice. We have some sith characters like Darth Sion, Darth Nihilus , Darth Traya who are all members of the Sith Triumvirate which would be a good fit since the Sith Temple featured is very similar to Darth Traya's temple used in KOTOR.
When it comes to characters who could be alive and serve directly in the story line we have Grand Admiral Thrawn and Starkiller as possabilities.
A special mention goes to Darth Bane who was made canon in Star Wars Clone Wars.