When writing the Fandex, I would suggest combining all the ideas above rather than making the Fandex specific to Guard only. Here is what ran through my head when I saw this - you don't want to just do Dark Eldar as they lack the room for cool conversion work. The Witch Hunters wouldn't make sense, as they would most likely be hunting these pirates.
So heres what I am thinking (in my slightly intoxicated state of mind) - make the fandex more scenario based, such as the fan based Killzone rules. Have it revolve around an extremely important planet, with an abundance of resources important to the Imperium, needed by the bands of pirates, and wanted by the greedy forces of chaos simply to make sure they dont go to the imperium. Dark Eldar have xenos bands of pirates, the Imperium has both renegade Guard pirates as well as Witch Hunter's who seek to track down the supposed pirates to cut down competition over resources, and chaos has to be throw into the mix because it works well with everything.
Oh, didn't read the above post - Orks definitely have to be there to, they don;t really know what their fighting for, they just like the shiny metal everyone wants. They also have a lot of cool pirate looking bits and models, and Flash Gitz are cool