Whether or not existing EU characters are included in any way probably depends on the rights situation. If Zahn holds the rights to Thrawn and Mara Jade etc., then there's probably no chance. If the rights are held by LucasArts, that might
be a different story.
Apparently plans for a third trilogy began circulating "last August"
, so it's likely that the initial groundwork has already been laid. And with the original story idea almost certainly coming from Lucas... my guess is that all the post-RotJ
EU material is just going to be cut off at the knees, the same way everything that was written about the Clone Wars prior to the release of the PT was.