I think it would be a Zoan, model Seagull. I know this sounds stupid for possibly the strongest character in the series but I think it fits him.
According to Luffy(and from Rayleigh's reaction Roger as well) the Pirate King is the person with the most freedom. Birds in general have always symbolized freedom because of the ability of flight and One Piece is no different. Seagulls are the epitome of the representation of freedom in OP imo, The first image in Romance Dawn is Luffy and a gull.
The first scene in the anime is an aerial shot of a seagull flying while talking about Roger. Here we see some of the best pirates, with jewels having a good time and seagulls
The Marines', the force restricting the freedom in OP, symbol is a seagull with a wrench. Wrench's are used to tighten things. Essentially there logo is saying we are locking down freedom.
Gol sort of looks like and sounds like, depending on pronunciation, Gull.
So to recap, PK=most freedom, Seagulls represent freedom in OP, his name sounds like Gull(sort of)
All that said I don't think Roger actually had a DF, but if he ends up having one I think this may be it.
According to Luffy(and from Rayleigh's reaction Roger as well) the Pirate King is the person with the most freedom. Birds in general have always symbolized freedom because of the ability of flight and One Piece is no different. Seagulls are the epitome of the representation of freedom in OP imo, The first image in Romance Dawn is Luffy and a gull.
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Gol sort of looks like and sounds like, depending on pronunciation, Gull.
So to recap, PK=most freedom, Seagulls represent freedom in OP, his name sounds like Gull(sort of)
All that said I don't think Roger actually had a DF, but if he ends up having one I think this may be it.