You know what kind of accessory would be awesome to have? Elemental Emitters :D Flame emitters have fire coming off of them, Frost has a cold misty effect and Shock has a sparking/Electrifying effect.
Want a mass with flaming eyes? Put a Flame Emitter on each eye :P
As said above, accessories that produce elemental effects like fire, mist, electrical, smoke etc.
This would pave way for designs even more unique that just with accessories alone! Battle damage for scenic photos could be created easily with this feature and the effects could be put onto weapons too for added effect. Want a custom flamethrower? Put a fire accessory in the barrel and boom! Want a smoking barrel? Put a smoke accessory in the barrel, you got it!