What is Canon?
Broadly speaking, canon is the official story line of a franchise. It is very common for a series of movies or a TV show to have content (usually books or comics) that do not directly tie in with the official history and universe of the original product.
"Star Wars" is unique, as its Expanded Universe series of novels used to share canon with its movies and TV shows. However, "The Clone Wars" and "Rebels" animated series have changed that, canonizing bits and pieces of the EU. Here are my picks as to what should be included in the official story line.
Including Xanatos would canonize most of the Jedi Apprentice series of books, which could be adapted well into an animated series down the road. Xanatos was a former apprentice of Qui-Gon Jinn, who turned rogue.
Xanatos was a cunning adversary, both for Qui-Gon and a young Obi-Wan. He didn't necessarily play around, as he was involved in a plot to destroy the Jedi temple and plotted an attempt to take Master Yoda's life to strike fear into the order.
Notable Quote: "You destroyed everything I loved. You destroyed me that day, Qui-Gon. Yet I was reborn. Stronger, wiser. I have surpassed you." - Xanatos to Qui-Gon Jinn*
Admiral Daala held a commanding presence amongst the Imperial fleet during the days of the New Republic. Though part of her canon backstory would have to be changed (she would have to be introduced as either part of the First Order or as a third party), she's an Imperial officer who's fantastic at destroying things. If the new movies decide to use the Sun Crusher (which does exactly what the name suggests) Admiral Daala would be a essential to have on board.
Notable quote: "My skills came to the attention of Moff Tarkin, who journeyed to Carida so that he could meet the mysterious individual who had developed such innovative tactics.[…]Tarkin was astonished to learn I was a woman and even more astonished to see that I was a lowly corporal working in the kitchen."
3. Jorus/Joruus C'baoth
Te original Jorus C'baoth died long before Episode I in Tomothy Zahn's "Outbound Flight." He was cloned, and was put into Emperor Palpatine's service.
The Clone, Joruus C'baoth, was mentally unstable, and dabbled in the dark arts. He was instrumental in the "Thrawn" trilogy and would make a good adversary in the new canon of "The Force Awakens." If anything, the events of "Outbound Flight" are worth bringing this jedi/ dark clone into the fold, as it deals with what lays beyond the galaxy far far away.
Notable Quote: "So at last you have come to me. I knew you would. Together we will teach the galaxy what it means to serve the Jedi."
A fan favorite, Revan walked the fine line between the light and the dark side of the force both as a jedi knight, and a Sith Lord. He is the perfect mentor figure for Kylo Ren, who struggles to balance these two sides out.
With "Star Wars Rebels," canonizing Malachor (an important location in "Knights of the Old Republic," the video game in which Revan originates from), it's not too far fetched that Disney would bring back this legend in some fashion.
Even if he doesn't appear in "Rebels" or "The Force Awakens," a one-off Revan movie would do wonders at the box office.
Notable Quote: "Revan had many Masters, Zhar, Dorak, Master Kae before Kae left for the Wars. Toward the end of his training, he sought out many to learn techniques. It is said that he returned to his first Master at the end of his training, in order to learn how he might best leave the Order."―Mical
1. Admiral Thrawn
"Star Wars" needs strong villains, and Thrawn was arguably one of the most cunning they had. There is a long period of time in between Episode VI and "The Force Awakens" this baddie could make an appearance. It would also be a welcome surprise to see this admiral step up where Hux, Phasma, and Kylo Ren failed. General Hux might be good at yelling at stormtroopers, but Thrawn is capable of crafting a master plan capable of shattering the New Republic, bringing Finn and Rey to their knees.
Notable Quote: "One can concentrate so closely on the words of a sentence that one thereby misses the meaning. As can happen in any area of life. You must never lose focus on the larger landscape."
*quotes were selected with the help of the Star Wars wikias.
Aether on January 21, 2019:
Mara Jade should be made Canon as well. Although I'm afraid Disney wouldn't picture her properly anyway, so I'm probably glad she stays in my heart the way I remember her since I was a kid. :)