Re: the Search for Strawhat's New Nakama...
Or: He could go with the SHs, who are going to Laboon as well...sorta. Well, most of them are. Chopper, Robin, and Franky probably don't give a fuck, if they weren't such good people.
The problem is, he might not wanna wait. It's been 50 years. And Brook doesn't know that Laboon is still waiting for him, and that will affect his decision alot, since he'll know he doesn't have to rush back as fast as he can because Laboon might have given up or be about to give up. Or maybe he'll sail off after Laboon before the Straw Hats have a chance to tell him, and they'll meet up with both Brook and Laboon at the end when they come full-circle around the Grand Line. I can see that happening. But my point is, it's still very uncertain that Brook will join. Vivi decided not to join because she felt that she had other responsibilities that needed her more and that she had to deal with for the good of the many, which outranked becoming a Straw Hat for herself and the crew. Brook may choose the same thing, deciding that showing Laboon that he didn't abandon him and keeping his promise may be more important then sailing with the Straw Hats, even though they also are trying to get to Laboon.
And besides, I don't want Brook to join. I don't know how he'd ever be able to leave the ship without scaring anyone who saw him, unless Chopper made him fake skin or something, which may be possible. If he joins, I suppose something like that will have to come about. But I think that it would be more emotionally impacting and true to Brooks character for him to decide that fulfilling his promise as fast as possible is more important than sailing with his friends, the Straw Hat Pirates. If he thinks that death isn't an apology, I doubt that he'll think that a need for companionship is one.
Just thought of this, though: What if Luffy convinces him that Laboon would WANT him to sail with them. Assuming it comes to that, and I'm sure it will, would Brook accept that argument? Or would he still decide to go back to Laboon on his own in the scenario I outlined before? (The one where he thinks the promise is more important.)