Maybe a marine ship that can pass through the calm belt will show up to deliver more slaves? I'm sure robin is hiding her devil fruit ability until she finds out all that she can about what's going on at the island, and then she'll make quick work of the guards.

It appears Hina arrived as well along with Jango and Fullbody. I'd like to see her do something awesome. I'm sure the giant guy isn't a marine admiral though...he looks like someone who'd act tough but then lose after 2 seconds against any of the stronger strawhats.