Most of the times Gaia uses, as base for the collectibles or cash shop, themes that involve some mystical and mithological inspiration.
Having that said, I think that
"Crystal Skulls" could be a good theme that could develop - besides an obvious crystal skull for you to hold or use in the place of your head - interesting crystal itens as the full skeleton, or just limbs of it, and maybe crystal clothing and hair/wig.
* About the crystal skulls:
There is a legend that says that are 13 skulls around the world, that in a certain time all of them will be reunited to reveal something for the world. Right now, apparently 5 or 6 have been discovered. The most famous is the Mitchell-Hedges skull. There are versions in trasparent color, in rose quartz and etc. For more information you can check
World Mysteries website and
Wikipedia (theres a great pic there of the skull), otherwise, this will become a huge post.