You can never go wrong with skulls and swirly flame-like stuff. Why not do both? Perhaps the swirls on the nose and front tips of the wings and the skulls where ever looks good? I know these were test models but you should, IMO, try to define the swirls as either \"decorative swirls\" or \"flames\", swirls would need to have more black showing through and between them, the swirls themselves should look less \"whispy\", more solid and defined...the flames would need a bit more definition in places and more termination points, forked tongues, etc (again, just my opinion). It doesn\'t look like you\'d want to do stylized flames, so I wont comment about those, plus I hate those...
I like the off-black colour too, more so than strait black, just throwing my vote out there.
P.S. Lovin\' this thread, great stuff, very interesting.