Subelements work like this.
Bending.(element).(subelement)
The subelements are:
Bloodbending
Healing
Icebending
Plantbending
Sandbending
Metalbending
Lavabending
Combustionbending
Lightningbending
Spiritualprojection
Flight
I might make a little addon plugin that gives you a list in game starting off with the subelements. It's like the only thing I'd be able to do coding wise right now.