Well for starters, it's fairly obvious someone dies. The way he thinks back all the way to the first time he met Leia indicates that it's probably someone who's been with them for a long time, since Endor at least. Here's a list of likely candidates:

Leia: Not very likely. For one thing, if Leia had just died, Han's thoughts would probably be either unintelligible or inappropriate for this kind of novel. Besides, she is way too important to kill off, at least in the first book of the series.

Luke: Almost definitely not. Del Rey's summary of the books mentions a focus on him and the kids working toward a, you guessed it, new jedi order. This also rules out Jacen, Jaina, and Anakin.

Wedge: Somewhat likely in the sense that he's a major character, but not really important enough. Also, he and Han never really seemed to have the kind of relationship that Han had with some of the other candidates. I would say second most likely, or a close third.

Chewbacca: Wedge's opponent for second most likely. As close to Han as they come, plus not really as important to the plot as some others. He'd be sorely missed, though.

Lando: My money's on him. He's been around for a while, is good friends with Han, AND hasn't been terribly important to most novel plots so far. Not to mention that the death of an old smuggling buddy would definitely give Han the sort of pause we witnessed in the excerpt. This would also add a touch of irony, as Lando was originally supposed to die way back at Endor, anyway.

Well, there you have it. This is all speculation, of course, and I may be way off. If you have any other ideas, contact me and I'll put 'em up.