http://naruto.wikia.com/wiki/Nature_transformation
and all the Chakra Natures
looking at the image on this page, did the manga never reveal the last two none filler chakra natures ?
Imo
Swift Release would make since for the nature combination of Wind and Lightning that is missing
but the
Lightning and Fire one , the only things i can think of is it being like a Light nature or a Plasma Nature
also of the filler ones imo the Dark Release makes no since to have an element type.
while Crystal Release imo would be a triple combination of Water Fire and Earth , since its weak to lightning and all Water Fire and earth makes the most since, like how Dust in Fire Earth and Wind
Steel though would probably have to be Earth Fire and Lightning