We've seen many EU characters re-integrated into the new continuity with some adjustments.

-Quinlan Vos. His basic skill-set is still there, and I think he still trained Aayla. Elements of his clone war arc were used in Dark Disciple, but heavily re-written.

-General Cracken. Pretty easy one to do, given that he was a fairly minor character anyway. Also, he was retroactivally identified as one of the Falcon guys in ROTJ who don't do that much except say "pressure steady" (There's a cool deleted scene where they man the cannons on the blu-ray though).

-Thrawn. His basic backstory is intact, although the whole thing with the Outbound Fleet project is pretty much scrubbed, and his activity is pretty much confined to the near-OT era with his current novels and REBELS. Although I suppose a future project could use him in some capacity.

However, should more be added?

Some tough ones but could work with some altering:

-Mara Jade. I can see her perhaps as a Jedi at Luke's ill-fated academy, or as an Imperial operative but it'd be hard to fit in the whole "Luke's wife" thing.

-Xizor. Marvel's already used the "Vader's rival" card a few too many times already.

-Talon Kaarde. Would be really easy I think, although Hondo's kind of taken his place as the whole uneasy underworld ally.

-Borsk Fey'la. We don't have much idea of how the New Republic worked between Jaaku and TFA apart from the Bloodline novels. I don't think we've even been given a canon Bothan look yet (The original EU sort of drew them inconsistently, as either horses, goats, dogs, or long-haired elves depending on the artist).

-Sith Lords (Kun, Sadow, Revan etc) or ancient Jedi (Bastila, Ulic etc). Apart from Momin in Darth Vader and of course Bane, the history of the Sith before Plageuis is largely vague, ditto with the ancient Jedi, although there are rumors of that being the subject of the next trilogy. Curiously, numerous Sith Lords have been named in recent canon books Master and Apprentice and Dooku Jedi Lost.

-I suppose the Vong could be integrated in some way, but with their threat heavily reduced like the Noghri were (Minus of course their whole Vader backstory and guardianship of Leia). Just as maybe just a weird alien race?

As for impossible, it's pretty much:

-The Solo kids, for obvious reasons.

-Ditto with Ben Skywalker.

-Although Palpatine's laugh is in ROS, somehow I don't think we're likely to see the Clone Emperor.

-Most of Luke's Jedi Order. I suppose you can still have Kyle Katarn as a rebel mercenary/ex-stormtrooper, but you'd have to ditch the Death Star plan stuff along with probably the Jedi element.