Ok, so we literally have a whole galaxy to choose from, but here are a few thoughts from me to get the ball rolling... Welcome everyone's suggestions.
Yoda and Ki Adi Mundi - these two are just sorely missing.
Wedge, Crix Madine, Garindan, Major Panno, Nien Nunb, Capt. Panaka, General Grievous - there are so many characters that could be brought in. Grievous is a must, but we could also have species along the same lines as the generic Jawa... Defel, Shistavanen, Gammorean, are just a few examples of some really good ones
Bossk, Dengar, 4-LOM and Zuckuss - how cool would it be with an ally's help to play the original Bounty Hunters!
Even Piell, Oppo Rancisis, Yarael Poof, Yaddle, Depa Billaba, etc - so I know there are already a plethora of Jedi, but this is Star Wars.
Aurra Sing, Dr. Evazan, Ponda Baba, Klaatu, Nikto, Barada - ok, so I added the last three to doft my cap to a Sci-fi classic, but wouldn't Sing be brilliant?
Vuffi Raa, Grand Admiral Thrawn, Rukh - let's not leave out the novels completely. I loved Lando's sidekick, and Noghris were bad ****.
Kybo Ren, Mungo Baobab, Jann Tosh - ok, it's maybe lower down the priority list, but I liked Baobab, and Ren was a funny bad guy... No worse than some of the stuff in Rebels!
Kyle Katarn, T3-M4, Dark Trooper, Tyber Zann - I love HK47, so here are some other old game characters that could be considered. Perhaps T3-M4 could have a repair ability for droids similar to that of healer. (Personally I'm not a fan of the healers in the game... Repairing a droid is a bit more realistic, so if I had my way I'd lose healers and introduce repair to balance out droids a bit more).
And lastly, as a gift to everyone, I'd like to see a community challenge where we could all get to pummel a team comprising of Jar Jar, Teek, Ziro, Princess Kneesa, and last but not least... Ackmena.