In my opinion, it's either the Black Beard Pirates or a completely random character which we'd never guess.
I also think that they are not from Germa, there would be no reason for the mysterious shadow if they were from Germa as we all kinda expect them to save their brothers… my bet is also BB pirates as it is their best chance to get a hold of BM's Road Poneglyph, assuming that BB also wants to reach Laugh Tale.