[Spoilers] The Shadow Figure is...

[shri3kbat]

I believe the shadowy figure is Jack, first of all Aokiji uses a bike to get across the sea and second he has no reason to get to Punk Hazard. Jack was mentioned in earlier chapters so it makes sense that he would be the one to get there after Doflamingo arrive and we don't know how powerful he is though i doubt he's as strong as Doflamingo.

All we know is this shadowy figure is a strong dude because only Aokiji and Doflamingo have been shown to get across the ocean like that. However Oda would probably not show Aokiji this early and it wouldn't be like him.

This man said he overslept which is a famous trait of Aokiji, Aokiji has made ice birds before, he has some sort of eyewear which Aokiji is always seen with. His face matches Aokiji's perfectly aswell
There's too much pointing to it being Aokiji for it to possibly being someone else..

 

[Skylar Knight]

Just to point it out, it's possible that Aokiji has a brother Jack just might be that brother, thus making him very similar to Aokiji in several ways.