I'd like to see one of three things:
1) The Old Republic made canon. It's far enough removed chronologically, I don't see why they couldn't have kept it. Not to mention some of the coolest characters come from that time period..
2) The Darth Bane trilogy. Still one of the coolest characters ever in the SW Universe, and creator of the Rule of Two, which is what laid the groundwork for Palpatine/Sidious, so I don't see how it isn't canon by default.
3) Probably most realistically... I'd like to see Quinlan Vos' character and EU adventure made canon. Friend of Obi-Wan's who struggles with the line between the light and the dark side of the Force.. who becomes an agent for Count Dooku, and later is redeemed.. Surviving Order 66.. and having taught Aayla Secura, (considered one of the best of the Jedi Order) he's clearly a qualified Master to help teach the next round of Force Sensatives...