Actually, each Sin subclass is going to have its own theme and all three imps will have a specific theme that carries over all the Sins. Marchosias is chimeric, Vepar is aquatic, and Gremory is aerial/arboreal.
Creeping Death, for instance, will have them adopt characteristics of different venomous ambush predators as well as their specific character design themes. (Marchosias -> rattlesnake, Vepar -> blue ringed octopus, Gremory -> black widow)
There's going to be a lot of design variety across all 14 subclasses.