Another thing I found that I think is interesting that I want to put out there is another scenario we were shown on Thriller Bark.
In Zoro's fight against Ryuma, Oda did something pretty uncharacteristic at the time. He did a fake out by showing Zoro presumably losing the fight, and then proceeded to show what actually transpired soon thereafter.
The thing that strikes me the most interesting is how the scene actually plays out. It starts off by showing Zoro falling off the rooftop to the ground, and then switches to Ryuma standing atop the roof above Zoro, while shrouded in smoke.
Here we have a similar situation with Zoro laying on the ground, and Skeletal figure standing above Zoro, shrouded in smoke. The issue is, is that we don't know what transpired prior to this scene. Much like how Oda started off the fight against Zoro vs Ryuma. We didn't find out until after the fact.
The last we saw of Brook and Robin was well before Zoro and King's fight finished. We know that CP-0 was stalled from pursuing them, and we have no clue where they ended up in that timeframe. This is something Oda can rectify in a mini flashback, which would once again be reminiscent of Zoro's battle with Ryuma on Thriller Bark.
And here's the kicker. What so happens to be occupying Ryuma's body at the time, causing him to be animated? Brooks shadow.